The FlatModel is essentially a proxy model keeping expansion and
filter state per ProjectTree(View). Using a Utils::TreeModel makes
it fast enough to allow recreation of the proxy structure on
structural changes simplifying the parent/child logic significantly.

The {Session,Project,Folder,File}Node hierarchy still is still primary
information and shared by all views.

Change-Id: Ic08180a19bda37908280ff30e0737d188ed93e92
Reviewed-by: Robert Loehning <robert.loehning@qt.io>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>