Blender Git Loki

Property Search: All tabs

This patch enables property search for all tabs in the property editor.
This is definitely the most "crazy" of the changes in terms of code, but
it's also where the new functionality gets good.

There is one piece of funcionality that wasn't obvious when I began the
implementation. In order to make interaction faster, if the editor's
current tab doesn't have a result, the search moves you to the next panel
that does. That way you can just press `ctrl-F`, search, and have the
result appear.

In order to keep the searching safe, to make sure the editor isn't influenced
by anything that happens while building the layout for the other tabs,
the space is duplicated and the new search is run in the duplicated
editor. This also helps isolate this code, which could be fairly invasive
otherwise. Only the layout pass is done in the other tabs, and it works
by just running the regular single tab property search (D8856) and
checking if any of the active panels in the tab match the search filter.

The search match status for every current tab of the property editor is
stored in a runtime field and them displayed later by dimming icons in
the tab selector panel to the left. The functionality for that dimming is
in D8858.

Note that there are currently some issues with the tool tab, where some
context variables end up being `None`. I'm curious if reviewers have any
advice if they encounter this.

**Future Improvements**
This patch does not addresss the performance aspects of searching every
tab. This shouldn't be too bad, but it would be good to make some changes
to improve this, ideally in separate patches. I have done some initial profiling,
and it looks like a significant portion of time is spent doing string comparisons,
so that may be the best place to start. Here are some ideas:
1. Use ghash instead of string lookups for panel types
2. Possibly only search in other tabs while editing search string. I would like
to avoid this though.
3. Look into using ED_region_tag_redraw_no_rebuild for some interactions
like panel dragging.

Differential Revision: https://developer.blender.org/D8859