Add: Debug tool to draw widget outlines. (#11401)

This is considered a developer tool and is controlled from the help menu (or default hotkey Ctrl-O).

This draws a white dashed outline around widgets. NWidgetSpacer and (unused) WWT_EMPTY widgets are also filled with check pattern to highlight them, as they usually indicate a design issue.

src/toolbar_gui.h

@@ -56,6 +56,7 @@ enum MainToolbarHotkeys {
 void AllocateToolbar();
 void ToggleBoundingBoxes();
 void ToggleDirtyBlocks();
+void ToggleWidgetOutlines();
 
 extern uint _toolbar_width;