Add keyboard shortcuts for rating

[CreamyCookie]

The proposed change

This existed at some point (before https://github.com/quodlibet/quodlibet/commit/36c6b1855dadb56ace4014b8cb3a748bfcae11d6).

There was Ctrl + 0-9, which is now used for changing the current browser.

Maybe (Ctrl + ) Alt + 0-9 could be used.

Benefits of this change

Rating is once again easy and doesn't require the mouse or custom scripts. (You can set ratings with quodlibet --rating=0.5 for current track)

Any issues this might cause

None, that I can think of. AFAICS people's shortcut changes would have priority.

[xubuild]

please make it global!