BugFix #84  -  Select TimeTrack row and press enter crash

- In CellRenderer_TimeTrack::activate_vfunc, Add initial test on event!=NULL
Merge pull request #97 from d-j-a-y/master

BugFix #84  -  Select TimeTrack row and press enter crash
Fix bug #543
http://www.synfig.org/issues/thebuggenie/synfig/issues/543