Maintenance Plan Actions - add session variables and expressions, more maintenance plan options to admins

Right now Create Action under maintenance plans is very limited to end user and admins for Plan actions.  Session variables should be incorporated into Database Maintenance Plans area so a user/admin can automate additional processes. Similar to how SQL Jobs work and scheduling. 

Ultimately I'd like new session group expressions, like LastHostConnectedEventTime > $XDAYSAGO wrapped into a action of deleting any support sessions based on it's own schedule.