Initial attempt at getting Qt DnD working with Enable.
Make Qt events match Wx more closely. Add DnD to zoomed_event_example.
Fix comments.
Fix old-style exception raising code.
Fix logic when TraitsUI isn't available.
Only accept() events if Enable handles them.
Remove use of handled as a flag - not needed.
Merge pull request #115 from enthought/feature/qt-drag-and-drop

Drag and Drop support for Qt