ReactiveHTML: Make it easy to create widgets that work as Panels widgets

[MarcSkovMadsen]

Sometimes I would like to create a Widget using ReactiveHTML. I would like to be able to use it in exactly the same way as Panel Widgets.

For example I would expect to be able to use .from_param method which I use very often. But I can't as a ReactiveHTML element does not have a .from_param method. If I try to copy the .from_param code from the Widget class it does not work either as a lot of the Panel code makes tests like isinstance(widget, Widget) and my component does not inherit from Widget as I don't know if I can just do this.