LibGUI: Add ScreenRectChangeEvent and deliver it to windows and widgets

Having to rely on GUI::Desktop's on_rect_change is quite limiting and a
bit awkward (continuing to use it would mean having to setup the
callback in every application using a webview) - we need a better way of
letting widgets know of a screen rect change automatically.

The specific use case will be IPWV/OOPWV which need to keep track of the
screen rect and notify the WebContent service of any change (which on
its own deliberately can't interact with WindowServer at all).

It'll also be useful for notification windows and the taskbar, which
currently both rely on the GUI::Desktop callback but will now be able to
listen and react to the event themselves.

Userland/Libraries/LibGUI/Widget.h

@@ -342,6 +342,7 @@ protected:
     virtual void drag_leave_event(Event&);
     virtual void drop_event(DropEvent&);
     virtual void theme_change_event(ThemeChangeEvent&);
+    virtual void screen_rect_change_event(ScreenRectChangeEvent&);
 
     virtual void did_begin_inspection() override;
     virtual void did_end_inspection() override;