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Property Search: Quick start and clear operators

`ctrl-F` to start the search is obviously necessary, but the clear
operator, `alt-F` requires some of explanation. First, it maps nicely
to the paradigm of "key to set, alt-key to clear," which makes it
unobtrusive. Second, it can be a quicker way to clear the search than
moving the mouse to the top. Finally, in the future, it could a reset
the panels to their expansion before the search started.

Differential Revision: https://developer.blender.org/D8857