#4722: Stage 1 creating proxyModel for table
#4722: Stage 2 creating proxyModel for table

- Crashes on deleting element if proxy is connected to tableView
- dataChanged() also emitted in filter if source emitted it
- updateMap should be called if something changed, it recalls everything
from model again
- bug about dragging mouse over scene (with object), then leaving scene,
then again in scene, then if want to leave, it crashes
#4722: Final stage creating proxyModel for table

- Now it is based on just QAbstractTableModel, not actual proxyModel,
but hardly connected to real model
- Uses signals/slots to recognize if something changed and reloads
everything, forming QMap of indexes again
#4722: Updated proxyModel to Content field