Support narrowing buffer selection with Alt+0-9 after using Alt+g or when onlyUnread is enabled, and show numeric overlays for Alt+0-9 keys on the buffers
Quick keys disappear only after releasing alt key

Other changes:
- 0.2s delay so they don't appear when pressing Alt+A etc
- don't appear when other modifier keys are pressed
Merge pull request #483 from glowing-bear/key-overlays

Show quick access keys on Alt keypress