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.
This fixes an assertion in TextEditor when changing the system theme,
since that would trigger a repaint request for the HTML preview widget
which may not have backing unless it's actually been used to perform
HTML (or Markdown) preview yet.
