WindowServer+LibGfx: Move title bar button layout to WindowTheme

Author: https://github.com/thankyouverycool
Commit: 208cb995ba
Pull-request: https://github.com/SerenityOS/serenity/pull/3303
Reviewed-by: https://github.com/awesomekling

Libraries/LibGfx/WindowTheme.h

@@ -59,6 +59,8 @@ public:
 
     virtual IntRect frame_rect_for_window(WindowType, const IntRect& window_rect, const Palette&) const = 0;
 
+    virtual Vector<IntRect> layout_buttons(WindowType, const IntRect& window_rect, const Palette&, size_t buttons) const = 0;
+
 protected:
     WindowTheme() { }
 };