Without this patch the item library gets into an
invalid state if the user enters edit mode
with an ative search filter.
When going back to design mode the item library
is completely empty.

This patch clears the filter, so the item library
gets properly populated.

Change-Id: I21f823b8db820589dedd6ba4e4855678fff49e53
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>