Added a brush reset operator so that a user won't need to reload the default blend to get back default brush settings
* New brush.reset operator, resets a brush based on the currently-selected tool
* Added UI button in the tools panel
TODO:
* Only resets sculpt brushes right now, other paint modes should be added
* Sculpt polish tool exists only as a Brush, not as a tool; I'd suggest we make it a tool so it can be reset to defaults too
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