Rename and preserve tags containing # so they can be used
Add basic collections management support (PICARD-84)

Adds a "Collections" submenu to the album context menu.

The code is mostly based on my GSoC branch from a couple years ago,
but I made some heavy changes.
Merge branch 'master' of github.com:musicbrainz/picard
fixup! Rename and preserve tags containing # so they can be used
Merge branch 'master' of github.com:musicbrainz/picard