QmlDesigner.ItemLibrary: the state of sections should be persistent

Once a section is collapsed it should stay collapsed until
it is expanded again.
Before this patch changing the document or going to text mode
did reset the state.

Change-Id: I5d3b4164cbef75276a2e0e92fb4bfa77292f73db
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>