Added an optional parameter to "CellToolbar.register_callback".

It lists the cell types the UI element must inserted in.
Hide the cell toolbar if there are no ui controls in it.
Hide toolbar of rendered text cells.
Why overriding a method if the new one is exactly the same of the one
in the base class?
Removed "cell_types" parameter from "select_ui_generator".

If you don't want to show the UI element in a cell use the
"register_callback" optional parameter.
Merge pull request #5627 from AlbertHilb/ConditionalCallbacks

Some improvements to cell toolbars