Fix missing adding of project nodes in treeview.
Store model ids in treeview nodes.
Store model type information in treeview nodes.
Store object pointer in treeview nodes.
Make it possible to retrieve and set flags via data methods.
Make it possible to retrieve validity via data method.
Add helper methods for getting and setting properties of treeview items.
Cleanup.
Merge branch 'treeview-items' into develop