'Explorer': Hide seldom used files and folders

[jheyne]

I work in Dart. 99% of the time the only folders I want to see are 'lib' and 'test'. The rest are boiler plate and config -- the clutter is hard on the eyes and mind. It would be nice to hide the stuff I seldom use.

One possible implementation:

1. Select multiple files and folders [already implemented]
2. Add item to pop-up menu: 'Create virtual folder'
3. Represent all the selected files in a single virtual folder labeled, '${fileCount} hidden files'
4. The virtual folder could be toggled open and closed just like a real folder