Fix some issues with AED.
 - When editing an action, you can now add more triggers to it.
 - Trigger Item dropdown now only contains the triggers that are compaible but not yet set rather than, all triggers except the last one you added.
 - If only a single TriggerGroup exists, automatically select it.

Change-Id: I16b7da978fc9acc99085036a660c10617d5d1bcb
Reviewed-on: http://gerrit.dmdirc.com/2631
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager