Widget.loading scroll-related glitch fixes

[davep]

This PR addresses a couple of issues when using Widget.loading to show a loading indicator on a widget:

• If the scrollbar gutter was fixed, it would cause some of the background widget to show through.
• If the underlying widget was scrollable, and was scrolled down some way, the loading indicator would either be partially visible or not visible at all.

Fixes #3733.

[willmcgugan]

Thinking about this some more. Why can't the loading overlay simply cover the scrollbar and gutter?

That might negate the need for this _widget_state weak dict.

[davep]

As per in-person discussion, handing this off to @willmcgugan for a wee play to consider some alternative methods of doing this type of overlay.

[davep]

Closing in favour of #3816.