A few instances where groups hang around, and block creation of pointless duplicate groups

Fixes available in lp:~stephen-j-boddy/terminator/dev revision 635 & 636. (I'll not bother attaching as patches unless you (Chris) say otherwise. It's a bit of a chew, and you can get them from the revision page of the above branch anyway.)

Issue 1:
Expected behaviour:
No duplicate groups in the Group menu

Actual behaviour:
Enter the same thing twice in the Create window, and you have two entries. It doesn't break anything

Issue 2:
Expected behaviour:
When the group is no longer used by any terminals it should be removed by the hoover function.

Actual behaviour:
Occasionally the groups will hang around in the menu after the last assigned terminal has closed or left the group. One instance where the hoover statement is missing, another where it is nested one level too deep. These also don't break anything.