Various answer queue improvements.

Created new "queue" menu.

Implemented hiding of closed questions from the queue (Issue #4). This
option can be toggled via the queue menu or using CTRL+H. (See also
Issue #15)

Implemented alternative sorting of the queue. The queue can now be
sorted by timestamp (as before, default), by question number, or by
status. The sort method can be change via the queue menu or using the
following mnemoics:

Timestamp: CTRL+T
Question Number: CTRL+Q
Status: CTRL+S