Changed aroundtime to a time window (feature #1400)

@ProfYaffle & @adamsutton I rebased and cleaned up my version of the patch as discussed in the old pull request.
I really don't like the way this display is working, if I try to select 00:00 then it just rejects the selection. Which is possibly marginally better than before where it would simply disable the field, but its not right.

Just provide a simple 00:00 to 23:45 selection (you could even stick an extra 00:00 on the end). You logic already copes with this.

You also need a "disable" option (-1) to disable the around time feature. In an ideal world this would disable both start/end selections, but in the grid structure we've got I think that's just a step too far.

Just assume that if either value is set to disabled then its disabled (best to update the internal config to reflect that).