improve adding applets through dragging responsive

--fix all events identifications from DragMove and
DragExit etc. in containment::qml part and all drags
flags are calculated only at DragEnter! This fixes
the DragMove signaling and all DragMove signals are
sent correctly.
It needs to be fixed also for Tasks plasmoid because
every time the mouse hovers the Tasks Plasmoid then
the DragMove signals are NOT SENT as they should.