FontEditor: Add undo and redo commands

Author: https://github.com/thankyouverycool
Commit: 44cd121e30
Pull-request: https://github.com/SerenityOS/serenity/pull/6563

Userland/Applications/FontEditor/GlyphEditorWidget.h

@@ -35,6 +35,7 @@ public:
     void set_scale(int scale);
 
     Function<void(int)> on_glyph_altered;
+    Function<void(bool finalize)> on_undo_event;
 
 private:
     GlyphEditorWidget() {};