LibGUI: Add Widget focus policies

Every widget now has a GUI::FocusPolicy that determines how it can
receive focus:

- NoFocus: The widget is not focusable (default)
- TabFocus: The widget can be focused using the tab key.
- ClickFocus: The widget can be focused by clicking on it.
- StrongFocus: Both of the above.

For widgets that have a focus proxy, getting/setting the focus policy
will affect the proxy instead.

Libraries/LibGUI/AbstractButton.cpp

@@ -35,6 +35,7 @@ namespace GUI {
 AbstractButton::AbstractButton(const StringView& text)
     : m_text(text)
 {
+    set_focus_policy(GUI::FocusPolicy::StrongFocus);
     set_background_role(Gfx::ColorRole::Button);
     set_foreground_role(Gfx::ColorRole::ButtonText);