FIX Iterating over list of editors while removing them.

That was not going to work...
Merge pull request #76 from enthought/fix-destroy-editors

FIX Iterating over list of editors while removing them.
FIX: Drag starting at the gray bar created an invalid drag_obj.

Instead, the drag_obj should be None, since this is not a valid drag.
Using 'px' for font-size instead of 'pt'.
Merge pull request #77 from agrawalprash/master

FIX: Drag starting at the tab bar (not on any tab) created an invalid drag_obj.
BUG: Qt: Destroying Window does not destroy native control.

Closing/destroying the window did not destroy the native control
which may appear as a memory leak.
This also affects TaskApplication, which leaves around TaskWindow
references even after the window is closed.
Merge pull request #79 from pankajp/qt-destroy-window-control

BUG: Qt: Destroying Window does not destroy native control.
Destroy SplitEditorAreaPane empty_widget when removing.

QTabWidget's removeTab only removes the tab from the tabs list,
so we need to explicitly destroy the empty_widget.
Merge pull request #80 from pankajp/split-editor-area-destroy-empty-widget

Destroy SplitEditorAreaPane empty_widget when removing.
TaskAction: Disconnect task dependent listeners for action.

This commit sets the TaskAction's task to None in .destroy() method,
so that task dependent trait listeners are disconnected.
Unless this is done, TraitChangeNotifyWrapper wrappers are
leaked on creating and deleting TaskAction instances which have
listeners on TaskAction dependent properties.
Merge pull request #81 from pankajp/task-action-destroy-listeners

TaskAction: Disconnect task dependent listeners for action.
Ability to add custom open dialog for split editor area empty widget.

Applications need ability to open custom open dialogs (easier than
writing the entire empty_widget), since the editor area does not
know the directory to default to and the file filters to attach
to the dialog.
Merge pull request #82 from pankajp/tasks-split-editor-area-empty-widget-custom-open-dlg

Ability to add custom open dialog for split editor area empty widget.
REL: Update version for 4.3.0 release.
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
replaced deprecated PyProtocols 'implements' by new traits.adaption 'provides'
fixed one last occurrence of implements
fixed ETS import convention