GUI2/Settings: bunch of cleanup and minor refactoring

Notable changes:
* get_window_builder now returns a const reference instead of an iterator.
* gui_definition and builder_window (not the one in window.cpp) now take a config reference directly
  and call read() from their ctors instead of requiring that be called manually.
* Added some more documentation.
* Made variable names more accurate.
* Removed some unused static variables
* Removed the unused non-class load_widget_definitions function (the gui_definition member of the same
  name remains.

src/gui/core/window_builder.cpp

@@ -125,9 +125,8 @@ window* build(CVideo& video, const builder_window::window_resolution* definition
 
 window* build(CVideo& video, const std::string& type)
 {
-	std::vector<builder_window::window_resolution>::const_iterator definition
-			= get_window_builder(type);
-	window* window = build(video, &*definition);
+	const builder_window::window_resolution& definition = get_window_builder(type);
+	window* window = build(video, &definition);
 	window->set_id(type);
 	return window;
 }
@@ -251,11 +250,8 @@ builder_widget_ptr create_builder_widget(const config& cfg)
  *
  *
  */
-const std::string& builder_window::read(const config& cfg)
+void builder_window::read(const config& cfg)
 {
-	id_ = cfg["id"].str();
-	description_ = cfg["description"].str();
-
 	VALIDATE(!id_.empty(), missing_mandatory_wml_key("window", "id"));
 	VALIDATE(!description_.empty(),
 			 missing_mandatory_wml_key("window", "description"));
@@ -268,8 +264,6 @@ const std::string& builder_window::read(const config& cfg)
 	{
 		resolutions.emplace_back(i);
 	}
-
-	return id_;
 }
 
 /*WIKI

src/gui/core/window_builder.hpp

@@ -150,12 +150,14 @@ typedef std::shared_ptr<const builder_grid> builder_grid_const_ptr;
 class builder_window
 {
 public:
-	builder_window() : resolutions(), id_(), description_()
+	explicit builder_window(const config& cfg)
+		: resolutions()
+		, id_(cfg["id"])
+		, description_(cfg["description"])
 	{
+		read(cfg);
 	}
 
-	const std::string& read(const config& cfg);
-
 	struct window_resolution
 	{
 	public:
@@ -203,6 +205,8 @@ class builder_window
 	std::vector<window_resolution> resolutions;
 
 private:
+	void read(const config& cfg);
+
 	std::string id_;
 	std::string description_;
 };

src/gui/widgets/helper.cpp

@@ -33,13 +33,15 @@ static bool initialized_ = false;
 
 bool init()
 {
+	const size_t start = SDL_GetTicks();
 	if(initialized_) {
 		return true;
 	}
 
 	load_settings();
 
 	initialized_ = true;
+	std::cout << "GUI2 init took " << (SDL_GetTicks() - start) << " ms" << std::endl;
 	return initialized_;
 }