Make "Quit" the only mandatory widget on the title screen

Everything else is optional, so the UX can be changed simply by removing
buttons and the tips widget from the title_screen.cfg file.

src/gui/dialogs/title_screen.cpp

@@ -75,37 +75,34 @@ namespace dialogs
  * This shows the title screen.
  *
  * @begin{table}{dialog_widgets}
- * tutorial & & button & m &
+ * tutorial & & button & o &
  *         The button to start the tutorial. $
  *
- * campaign & & button & m &
+ * campaign & & button & o &
  *         The button to start a campaign. $
  *
- * multiplayer & & button & m &
+ * multiplayer & & button & o &
  *         The button to start multiplayer mode. $
  *
- * load & & button & m &
+ * load & & button & o &
  *         The button to load a saved game. $
  *
- * editor & & button & m &
+ * editor & & button & o &
  *         The button to start the editor. $
  *
- * addons & & button & m &
+ * addons & & button & o &
  *         The button to start managing the addons. $
  *
- * cores & & button & m &
- *         The button to start managing the cores. $
- *
- * language & & button & m &
+ * language & & button & o &
  *         The button to select the game language. $
  *
- * credits & & button & m &
+ * credits & & button & o &
  *         The button to show Wesnoth's contributors. $
  *
  * quit & & button & m &
  *         The button to quit Wesnoth. $
  *
- * tips & & multi_page & m &
+ * tips & & multi_page & o &
  *         A multi_page to hold all tips, when this widget is used the area of
  *         the tips doesn't need to be resized when the next or previous button
  *         is pressed. $
@@ -117,10 +114,10 @@ namespace dialogs
  *         The source (the one who's quoted or the book referenced) of the
  *         current tip. $
  *
- * next_tip & & button & m &
+ * next_tip & & button & o &
  *         The button show the next tip of the day. $
  *
- * previous_tip & & button & m &
+ * previous_tip & & button & o &
  *         The button show the previous tip of the day. $
  *
  * logo & & image & o &
@@ -159,7 +156,11 @@ static void register_button(window& win, const std::string& id, hotkey::HOTKEY_C
 		win.register_hotkey(hk, std::bind(callback));
 	}
 
-	connect_signal_mouse_left_click(find_widget<button>(&win, id, false), std::bind(callback));
+	auto b = find_widget<button>(&win, id, false, false);
+	if(b != nullptr)
+	{
+		connect_signal_mouse_left_click(*b, std::bind(callback));
+	}
 }
 
 static void launch_lua_console()
@@ -266,30 +267,31 @@ void title_screen::pre_show(window& win)
 	//
 	// Tip-of-the-day browser
 	//
-	multi_page& tip_pages = find_widget<multi_page>(&win, "tips", false);
+	multi_page* tip_pages = find_widget<multi_page>(&win, "tips", false, false);
 
-	std::vector<game_tip> tips = tip_of_the_day::shuffle(settings::tips);
-	if(tips.empty()) {
-		WRN_CF << "There are no tips of day available." << std::endl;
-	}
+	if(tip_pages != nullptr) {
+		std::vector<game_tip> tips = tip_of_the_day::shuffle(settings::tips);
+		if(tips.empty()) {
+			WRN_CF << "There are no tips of day available." << std::endl;
+		}
+		for(const auto& tip : tips)	{
+			string_map widget;
+			std::map<std::string, string_map> page;
 
-	for(const auto& tip : tips)	{
-		string_map widget;
-		std::map<std::string, string_map> page;
+			widget["use_markup"] = "true";
 
-		widget["use_markup"] = "true";
+			widget["label"] = tip.text();
+			page.emplace("tip", widget);
 
-		widget["label"] = tip.text();
-		page.emplace("tip", widget);
+			widget["label"] = tip.source();
+			page.emplace("source", widget);
 
-		widget["label"] = tip.source();
-		page.emplace("source", widget);
+			tip_pages->add_page(page);
+		}
 
-		tip_pages.add_page(page);
+		update_tip(win, true);
 	}
 
-	update_tip(win, true);
-
 	register_button(win, "next_tip", hotkey::TITLE_SCREEN__NEXT_TIP,
 		std::bind(&title_screen::update_tip, this, std::ref(win), true));
 
@@ -404,16 +406,19 @@ void title_screen::pre_show(window& win)
 	// Quit
 	//
 	register_button(win, "quit", hotkey::HOTKEY_QUIT_TO_DESKTOP, [&win]() { win.set_retval(QUIT_GAME); });
+	// A sanity check, exit immediately if the .cfg file didn't have a "quit" button.
+	find_widget<button>(&win, "quit", false, true);
 
 	//
 	// Debug clock
 	//
 	register_button(win, "clock", hotkey::HOTKEY_NULL,
 		std::bind(&title_screen::show_debug_clock_window, this));
 
-	find_widget<button>(&win, "clock", false).set_visible(show_debug_clock_button
-		? widget::visibility::visible
-		: widget::visibility::invisible);
+	auto clock = find_widget<button>(&win, "clock", false, false);
+	if(clock) {
+		clock->set_visible(show_debug_clock_button ? widget::visibility::visible : widget::visibility::invisible);
+	}
 }
 
 void title_screen::on_resize(window& win)
@@ -423,25 +428,28 @@ void title_screen::on_resize(window& win)
 
 void title_screen::update_tip(window& win, const bool previous)
 {
-	multi_page& tips = find_widget<multi_page>(&win, "tips", false);
-	if(tips.get_page_count() == 0) {
+	multi_page* tip_pages = find_widget<multi_page>(&win, "tips", false, false);
+	if(tip_pages == nullptr) {
+		return;
+	}
+	if(tip_pages->get_page_count() == 0) {
 		return;
 	}
 
-	int page = tips.get_selected_page();
+	int page = tip_pages->get_selected_page();
 	if(previous) {
 		if(page <= 0) {
-			page = tips.get_page_count();
+			page = tip_pages->get_page_count();
 		}
 		--page;
 	} else {
 		++page;
-		if(static_cast<unsigned>(page) >= tips.get_page_count()) {
+		if(static_cast<unsigned>(page) >= tip_pages->get_page_count()) {
 			page = 0;
 		}
 	}
 
-	tips.select_page(page);
+	tip_pages->select_page(page);
 
 	/**
 	 * @todo Look for a proper fix.