FIX Accept drop event as soon as one of the handlers accept it.
Set active_editor only after UI preparations.

This way, and listener is notified when the UI
is ready.
Merge pull request #83 from enthought/split_editor_small_things

A couple of minor fixes to the SplitEditorAreaPane
merged upstream master
replaced deprecated PyProtocols 'implements' by new traits.adaption 'provides'
Merge branch 'adaption' into python3