SSP-2005 Fix course un-plan button

Appears to have been a regression from recent perf enhancements which
changed the type of SemesterPanel from Ext.Panel to Ext.grid.Panel,
which then invalidated assumptions about the view hierarchy in that
component.
SSP-2005 Re-enable planned course double click handler

Looks like this was a regression from recent perf changes that
migrated code from SemesterGridViewController to
SemesterPanelViewController. The itemdblclick event handler
registration just went missing.