Show Widget button appears multiple times in some cases

[SamLau95]

Sometimes we get multiple Show Widget buttons stacked on top of each other. This happens when we display multiple sets of widgets that each produce their own HTML hooks for nbinteract. We should limit the button to appear only once per cell, not once per output.

Related issue: https://github.com/clawpack/riemann_book/pull/128#issuecomment-372310916