LibGUI: Search for actions with a Shortcut instead of for KeyEvent

Instead of having widget/window/application create a shortcut from a
KeyEvent within their find methods, we'll just pass them a Shortcut
so that the "where to search" logic doesn't need to be duplicated
for different Shortcut types.

It also lets us handle invalid Shortcut rejection at a higher level,
with most things letting the caller be responsible for not searching
for actions with an invalid shortcut.

Userland/Libraries/LibGUI/Forward.h

@@ -60,6 +60,7 @@ class ResizeEvent;
 class ScreenRectsChangeEvent;
 class Scrollbar;
 class AbstractScrollableWidget;
+class Shortcut;
 class Slider;
 class SortingProxyModel;
 class SpinBox;