Disable Filter when appropriate

[GoogleCodeExporter]

Create rule, select root event, add an action, make sure action item is 
highlighted/selected, then click "add filter", it responds with "Sorry, you 
must select an event".  It should know to select that event automatically or 
disable the Add Filter button if action isn't selected.

Original issue reported on code.google.com by [email protected] on 2 Aug 2010 at 12:12