It's not possible to construct a Gfx::Bitmap with empty size. Let the
client know the new viewport rect and return before even attempting to
create new front and back bitmaps.
Also consider that we might have to paint the widget but not have a
front/back bitmap available (e.g. when only part of a scrollbar is
visible, and the inner rect is empty).
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.
