Formalizing interface to applications

[jlstevens]

With the addition of popup interfaces in Bokeh and Holoviews (via streams), we can now define widget panels that appear inline over the plots. This means we can now imagine, Annotator being used with the current interface (i.e. mostly based around __mul__) that automatically provides an inline input interface for defining fields without needing to worry about laying out widgets into a overall dashboard.

Talking together with @hoxbro, we think we ought to define an interface/base class that formalizes how Annotator uses potential panel applications. The idea is that the panel applications following this interface would define the widget popups that the annotator could use.

One thing to consider is whether the __mul__ based interface could actually show a more complex layout/dashboard as an additional option.