wip
to-one relations are being resolved in one select with the object.
Change MySQL column type of pixmaps to MEDIUMBLOB to allow bigger pictures.
Added QpSortFilterProxyObjectModel<T>::objects
Filtermöglichkeiten für Verträge und Rechnungen aktiviert.
Improved QpObjectListModel by:

- Moving most code to QpObjectListModelBase
- Adding setObjects, which allows you to use it as a "RelationObjectListModel" or something.
Merge commit '1e2566bff6ef5afb946508d60ed67f3f36c90194' into develop

* commit '1e2566bff6ef5afb946508d60ed67f3f36c90194':
  Squashed 'lib/QPersistence/' changes from 10e0363..6e52ace

Conflicts:
	lib/QPersistence/src/sortfilterproxyobjectmodel.h
Fix QPersistence compilation, when using QP_FOR_SQLITE
build QP_FOR_MYSQL