HorizontalProxyModel now inherits from QIdentityProxyModel which provides a more complete base implementation (maybe to much?)
for now it seems to do what we want, forward relevant signals which the abstract base didn't.
Issues: Properties table needs formatting and shows all rows as columns instead of values of a single row.
model is reset as this will clear all selections.
Unfortunatly the selection model does not trigger a selectionChanged when
the model is reset.
Close#16
