remove hack of loading workareas data engine in qml plugin
preparing to change QHash to QList in Workareas::Store

--this change is needed in order to support ordering
Workareas::Store change QHash to QList to support ordering
added Order field in the workareas data engine
implement QSortFilterProxyModel for tasks in desktop dialog

--dropped the old way to show tasks for specific workarea. I used
QSortFilterProxyModel that filters the tasks to show only the
relevant one.