Don't do any sizer work if the widget is not changing.
cleanup docstrings and old unused methods
Handle modality on qt windows.
Close any open sessions on event loop shutdown
Add a `destroy_on_close` flag to the base Window class.
Update documentation.
Undo the changes to end the sessions on event loop exit.

Use `Application.destroy` instead.
Add support for adding windows to a running session.

This commit also adds support for creating top-level windows which are
children of another widget. It also adds a more formal api for batching
actions on a session.
Prevent toplevel wx windows from being destroyed on close.

Update the Window class to toggle visibility to False when closed.
Add a popup windows example.
Merge pull request #244 from enthought/feature-session-windows

Feature session windows