Qt: implement download cancellation
Merge remote-tracking branch 'simark/v2-qt-dev' into v2-qt-dev

Conflicts:
	toutvqt/downloads_tablemodel.py
Merge remote-tracking branch 'eepp/v2-qt-dev' into v2-qt-dev

Conflicts:
	toutvqt/downloads_tablemodel.py
	toutvqt/downloads_tableview.py