More on palettes

Only one palette visible per activity.  A palette popup provokes the
popdown of the current visible one, if any.

If the invoker is a toolbar button, add a background box on top of the
palette, over the button.

If the invoker is not a toolbar button, the palette will popup in the
clicked position.
License under Apache 2.0
Merge https://github.com/sugarlabs/sugar-web

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Final commit to add radio and checkbox