Blender Git Loki

Property Search: Quick start and clear operators

This patch adds quick start and quick clear operators.

`ctrl-F` is obviously necessary, but the clear operator, `alt-F` requires
a bit of explanation. First, it maps nicely to the paradigm of "key to set,
alt-key to clear," which makes it pretty unobtrusive. Second, it can be
a quicker way to clear the search than moving the mouse to the top.
Finally, a future improvement I would like to make to property search
is adding the ability to reset the panels to their expansion before the
search started. Either escaping out of the button while text editing or
pressing `alt-F` would be the way to do that.

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