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/gui/style.h

@@ -66,6 +66,8 @@ void DrawDisplaySettings(ImGuiStyle *ref = nullptr);
 
 void TextWithSeparators(const absl::string_view &text);
 
+void DrawFontManager();
+
 static const char *ExampleNames[] = {
     "Artichoke",      "Arugula",          "Asparagus",    "Avocado",
     "Bamboo Shoots",  "Bean Sprouts",     "Beans",        "Beet",