calendar: Ensure clicked calendar item retains key focus

The date actors get destroyed and recreated on every date change which drops
key focus for the selected date. Restore key focus in such a case, but only
when the selected date was actually clicked. Whenever the next/prev month
buttons code is used (for scrolling, mouse click, or keyboard click), have
the corresponding button grab focus. Changing months currently causes the
calendar to update twice as the eventSource gets changed, so key focus gets
lost if it is on a date when the month changes.

https://bugzilla.gnome.org/show_bug.cgi?id=667434