Blender Git Loki

Property Search: Differentiate search filtered and inactive buttons

Currently there's no way to know if a button is inactive when it doesn't
match the search results, because they use the same 50% gray level.

This isn't a huge problem, but it could lead to confusion. This commit
uses a subtle solution, a 25% opacity when the button is inactive and
also filtered by search.

This requires flipping the meaning of the UI_SEARCH_FILTER_MATCHES
flag in the code, and also adding a widget_alpha_factor utility in
the widget code.

Differential Revision: https://developer.blender.org/D8975