LibGUI: Add FontsChanged event and deliver it to windows and widgets

Author: https://github.com/luk1337
Commit: b40d771512
Pull-request: https://github.com/SerenityOS/serenity/pull/8676

Userland/Libraries/LibGUI/Event.h

@@ -52,6 +52,7 @@ public:
         DragMove,
         Drop,
         ThemeChange,
+        FontsChange,
         ScreenRectsChange,
         ActionEnter,
         ActionLeave,
@@ -425,6 +426,14 @@ public:
     }
 };
 
+class FontsChangeEvent final : public Event {
+public:
+    FontsChangeEvent()
+        : Event(Type::FontsChange)
+    {
+    }
+};
+
 class ScreenRectsChangeEvent final : public Event {
 public:
     explicit ScreenRectsChangeEvent(const Vector<Gfx::IntRect, 4>& rects, size_t main_screen_index)