Blender Git Loki

Property Search: Set panel expansion when tab changes

This commit makes the panel expansion set based on the search results
when the active tab in the properties editor changes. The multi-tab
search patch (D8859) actually doesn't handle this because it uses a
different code path.

This feature uncovered a subtle but fairly significant issue with the
implementation of property search (More details in T81113). Basically,
the search needed multiple redraws to properly display the expansion of
panels based on the search results. Because there is no animation of
panel expansion when switching tabs, the problem was exposed only now.

With this commit, hiding of "search only" buttons and panel size
calculation happens in a single final step of the panel layout pass.
The "search only" layout root flag is removed. Instead every button
inside a panel header is in a single "uiButtonGroup" marked with a
specific "in header" flag, an idea which could be generalized in the
future.

Differential Revision: https://developer.blender.org/D9006