Ability to easily raise/lower widgets

[jarvisteach]

It would be useful to implement a feature to easily raise/lower widgets in the grid, when multiple widgets are in the same grid cell:

widget.lift()          # move to the top of the stack
widget.lift(another)   # move to just above another widget
widget.lower()         # move to the bottom of the stack
widget.lower(another)  # move to just below another widget

Originally mentioned in issue #603

[jarvisteach]

In next release