In the method dropTargetProc(long id, long sel, long arg0) the widget
was being recognised as label instead of the widget that actually has
the drop Target key. As soon as the first drop target key is acquired
the area is recognised as a Drop Target area and the Drag handlers are
added.

Also a new function called handleLabels( Control c, long cls) is
also added to bring the feature of droping on to the top of a label that
has an image view.This functions takes a control as a parameter to go up
till its parent to find the label which has an image view and add drag
handlers for that.
Fixes: https://github.com/eclipse-platform/eclipse.platform.swt/issues/649