ENH: Expose Box, Pos, Rect, Size in the API

These get used in Enaml files with reasonable frequency.
Merge pull request #239 from enthought/feature-expose-box-api

ENH: Expose Box, Pos, Rect, Size in the API
Merge remote-tracking branch 'upstream/master'
This adds a `root_object` method which addresses issue #240.
Merge pull request #243 from enthought/feature-root_object

This adds a `root_object` method which addresses issue #240.
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
Fix FileDialog lifetime bug from issue #217
Cut the 0.6.7 release
Merge remote-tracking branch 'upstream/master'
Update File_Dialog example, move to widgets sub-dir