Some improvements to cell toolbars

1. Added an optional parameter to ``CellToolbar.register_callback``.
If defined, the UI element will be inserted only in cells of the types listed in that parameter.

2. Hide a cell toolbar if it does not contain UI controls.

3. Hide the toolbar of rendered text cells.

As side-effect, I noted that in ``HeadingCell``, ``set_rendered`` was overridden by a method that was exactly the same of the base class one. So I removed it.