remove obsolete draggingStarted signals/slots

--draggingStarted signals/slots were used in order
to handle the active window dragging from empty areas.
The new approach ungrabs the mouse properly through
View relevant function and at the same time sends a
QEvent::Leave event to inform applets that the mouse
has left the view. This is enough in order to remove
any older draggingStarted workarounds and hacks.