Greatly improve the performance for the metadataselector class. By adding all
treeitems one-by-one the treewidget seems to trigger a sorting operation.
Adding many items like in the "Makernotes" tab resolves in a poor performance,
on my computer it takes nearly 10 seconds to add all the items, on a friends
computer it's even worse.

Adding the toplevel items parentless to a QList helps to improve the
construction of this tab.
It only takes up to one second now.