Fixed: Usage of enum-type properties.
merged QpAbstractObejctListModel into QpObjectListModel
Added convenience methods to QpSortFilterProxyObjectModel
Removed idiot-methods from QpCache.
Fixed a major bug in DAO, where removing objects would lead to nullpointers in the list returned by readAll
Relations now clear themselves, when a nullpointer is passed to relate()
Merge remote-tracking branch 'origin/geenen' into feature36-creationUpdateTimes

Conflicts:
	src/relations.cpp
	src/sqldataaccessobjecthelper.h
WIP update times