Refactor flags and add Font Manager to settings editor

Simplified `ExperimentFlags` by removing unused flags and updated
`Controller` to no longer inherit from it. Refactored `FlagsMenu`
to separate flag categories into individual methods for better
organization. Enhanced settings editor with a new "Font Manager"
tab and updated `DrawGeneralSettings` to use the new flag category
methods. Added `DrawFontManager` function for font management.

src/app/core/controller.h

@@ -26,7 +26,7 @@ namespace core {
  * This class is responsible for managing the main window and the
  * main editor. It is the main entry point for the application.
  */
-class Controller : public ExperimentFlags {
+class Controller {
  public:
   bool IsActive() const { return active_; }
   absl::Status OnEntry(std::string filename = "");