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 commit '29ee6216e82ca8a8711c99ecbde2a9fbfd22cca9'

Conflicts:
	lib/QPersistence/src/cache.cpp
	lib/QPersistence/src/cache.h
	lib/QPersistence/src/dataaccessobject.cpp
	lib/QPersistence/src/objectlistmodel.cpp
	lib/QPersistence/src/objectlistmodel.h
	lib/QPersistence/src/relations.cpp
	lib/QPersistence/src/sortfilterproxyobjectmodel.h
	lib/QPersistence/src/sqldataaccessobjecthelper.cpp
	lib/QPersistence/src/sqldataaccessobjecthelper.h
	lib/QPersistence/tests/tst_cachetest.cpp
Merge remote-tracking branch 'origin/master' into geenen

Conflicts:
	src/dataaccessobject.cpp
	src/relations.cpp
	src/sqldataaccessobjecthelper.h
	tests/tst_cachetest.cpp