MenuButton with custom Panel objects

[ahuang11]

At the current moment, I think MenuButton only supports strings, but I think with Bokeh floating panes now available, could it support other panes?

Imagine the menu items, combined with the contents of::

Or just think of a floating Card, i.e. the objects doesn't reposition other objects in the layout.

References:
bokeh/bokeh#13538
holoviz/holoviews#6168
https://panel.holoviz.org/reference/widgets/MenuButton.html
https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/popover