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/Button.cpp

@@ -38,6 +38,7 @@ namespace GUI {
 Button::Button(const StringView& text)
     : AbstractButton(text)
 {
+    set_focus_policy(GUI::FocusPolicy::StrongFocus);
 }
 
 Button::~Button()