Blender Git Loki

UI: Start towards property search in public branch

This is VERY WIP, but I will continue work in this branch. Implementation
starts with a few things:
- A pass at the end of the layout system to scan buttons based on a
search string set by the properties space. Buttons which don't fit
the filter are removed from the layouts.
- A pass after the filter to move buttons that are still in layouts
to a single column "search layout." There is currently a bug with
this that adds too much space in between items.
- An attempt to only show panels that haven't been completely
filtered by the previous two steps. This is quite buggy right now:
panels don't animate properly and sometimes it fails.
- An attempt at a keymap item "ctrl-F" to activate the search button.
"alt-F" should probably clear the button too.