Add move_to_front and move_to_bottom methods on window
Add wx functionality
Add sticky flag to windows
Add wx support for window stickiness
Add comment
Modify popup_windows example to show send_to_front and sticky functionality
Change sticky to 'always_on_top'
Minor cleanup for merge
Merge pull request #261 from enthought/feature-window-flags

Add send_to_[front|back] and sticky flag to window