Read/Import/Migrate files from Tilt Brush/Multibrush folder

[mikeskydev]

Users may have started on Tilt Brush or use Multibrush, but their files are in the Tilt Brush folder and aren't accessible from Open Brush.
We could do with some way to either read the Tilt Brush folder, or fully import/migrate contents to the Open Brush folder. The latter may be more ideal when we are forced to use SAF in the future.

[andybak]

#689 might help with the first option.

And general folder/file management tools can be something built on that UI later.