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/TextEditor.cpp

@@ -56,6 +56,7 @@ TextEditor::TextEditor(Type type)
 {
     REGISTER_STRING_PROPERTY("text", text, set_text);
 
+    set_focus_policy(GUI::FocusPolicy::StrongFocus);
     set_accepts_emoji_input(true);
     set_override_cursor(Gfx::StandardCursor::IBeam);
     set_background_role(ColorRole::Base);