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.

DevTools/HackStudio/FormWidget.cpp

@@ -33,6 +33,7 @@ namespace HackStudio {
 
 FormWidget::FormWidget()
 {
+    set_focus_policy(GUI::FocusPolicy::StrongFocus);
     set_fill_with_background_color(true);
     set_relative_rect(5, 5, 400, 300);