Revert "Remove an unnecessary layer of indirection"

This reverts commit cdec9b2. This commit broke selection of different
brushes in the Editor. Using a single pointer means changes to the brush pointer in editor_toolkit aren't
reflected in the action objects.

The double pointer isn't the cleanest code, but I'm not sure how to improve it.

src/editor/action/mouse/mouse_action.cpp

@@ -253,7 +253,8 @@ editor_action* brush_drag_mouse_action::drag_generic(editor_display& disp, int x
 const brush& brush_drag_mouse_action::get_brush()
 {
 	assert(brush_);
-	return *brush_;
+	assert(*brush_);
+	return **brush_;
 }
 
 

src/editor/action/mouse/mouse_action.hpp

@@ -165,7 +165,7 @@ class mouse_action
 class brush_drag_mouse_action : public mouse_action
 {
 public:
-	brush_drag_mouse_action(common_palette& palette, const brush* brush, const CKey& key)
+	brush_drag_mouse_action(common_palette& palette, const brush* const * const brush, const CKey& key)
 		: mouse_action(palette, key)
 		, previous_drag_hex_()
 		, brush_(brush)
@@ -235,12 +235,12 @@ class brush_drag_mouse_action : public mouse_action
 	editor_action* drag_generic(editor_display& disp, int x, int y, bool& partial, editor_action* last_undo);
 
 	/**
-	 * Current brush handle. Currently pointer to brush with full constness.
+	 * Current brush handle. Currently a pointer-to-pointer with full constness.
 	 * The mouse action does not modify the brush, does not modify the pointer
 	 * to the current brush, and we allow setting this pointr only once, hence
 	 * the three "consts".
 	 */
-	const brush* const brush_;
+	const brush* const * const brush_;
 };
 
 /**
@@ -250,7 +250,7 @@ class mouse_action_paint : public brush_drag_mouse_action
 {
 public:
 	mouse_action_paint(
-		const brush* brush, const CKey& key, terrain_palette& palette)
+		const brush* const * const brush, const CKey& key, terrain_palette& palette)
 	: brush_drag_mouse_action(palette, brush, key)
 	, terrain_palette_(palette)
 	{

src/editor/action/mouse/mouse_action_select.hpp

@@ -27,7 +27,7 @@ namespace editor {
 class mouse_action_select : public brush_drag_mouse_action
 {
 public:
-	mouse_action_select(const brush* brush, const CKey& key, empty_palette& palette)
+	mouse_action_select(const brush* const * const brush, const CKey& key, empty_palette& palette)
 	: brush_drag_mouse_action(palette, brush, key)
 	{
 	}

src/editor/toolkit/editor_toolkit.cpp

@@ -31,7 +31,7 @@ editor_toolkit::editor_toolkit(editor_display& gui, const CKey& key,
 	, palette_manager_()
 	, mouse_action_(nullptr)  // Will be set before this constructor ends.
 	, mouse_actions_()
-	, brush_(nullptr) // Will be set before this constructor ends.
+	, brush_(nullptr)
 	, brushes_()
 {
 	init_brushes(game_config);
@@ -62,11 +62,11 @@ void editor_toolkit::init_sidebar(const config& game_config)
 void editor_toolkit::init_mouse_actions(context_manager& cmanager)
 {
 	mouse_actions_.emplace(hotkey::HOTKEY_EDITOR_TOOL_PAINT,
-		std::make_shared<mouse_action_paint>(brush_, key_, *palette_manager_->terrain_palette_.get()));
+		std::make_shared<mouse_action_paint>(&brush_, key_, *palette_manager_->terrain_palette_.get()));
 	mouse_actions_.emplace(hotkey::HOTKEY_EDITOR_TOOL_FILL,
 		std::make_shared<mouse_action_fill>(key_, *palette_manager_->terrain_palette_.get()));
 	mouse_actions_.emplace(hotkey::HOTKEY_EDITOR_TOOL_SELECT,
-		std::make_shared<mouse_action_select>(brush_, key_, *palette_manager_->empty_palette_.get()));
+		std::make_shared<mouse_action_select>(&brush_, key_, *palette_manager_->empty_palette_.get()));
 	mouse_actions_.emplace(hotkey::HOTKEY_EDITOR_TOOL_STARTING_POSITION,
 		std::make_shared<mouse_action_starting_position>(key_, *palette_manager_->location_palette_.get()));
 	mouse_actions_.emplace(hotkey::HOTKEY_EDITOR_TOOL_LABEL,