gtk: Handle a null Gdk.Event passed to CellRenderer.StartEditing()
gtk: Handle a null Gdk.Event passed to CellRenderer.StartEditing()

Cherry-picked from the gtk-sharp-2.12 branch.

Gankov/gtk-sharp,akrisiun/gtk-sharp,sillsdev/gtk-sharp,openmedicus/gtk-sharp,openmedicus/gtk-sharp,antoniusriha/gtk-sharp,openmedicus/gtk-sharp,akrisiun/gtk-sharp,openmedicus/gtk-sharp,akrisiun/gtk-sharp,openmedicus/gtk-sharp,antoniusriha/gtk-sharp,orion75/gtk-sharp,Gankov/gtk-sharp,openmedicus/gtk-sharp,orion75/gtk-sharp,Gankov/gtk-sharp,antoniusriha/gtk-sharp,orion75/gtk-sharp,sillsdev/gtk-sharp,Gankov/gtk-sharp,antoniusriha/gtk-sharp,Gankov/gtk-sharp,antoniusriha/gtk-sharp,orion75/gtk-sharp,openmedicus/gtk-sharp,akrisiun/gtk-sharp,akrisiun/gtk-sharp,orion75/gtk-sharp,sillsdev/gtk-sharp,Gankov/gtk-sharp,sillsdev/gtk-sharp,sillsdev/gtk-sharp