Add and Remove data from BokehPlotPane and DeckglPlotPane

[lmcinnes]

It would be useful to be able to do en masse updates of plot panes. For example, if a user subselects some data, runs UMAP of the subset and wants to have the plot update to just that subset and new layout. The catch is that we need to handle all the things like size, apparent size, color etc. that are encoded into the javascript side info for the plot panes.

In essence this involves exposing a DataFrame param that contains all the data for rendering the PlotPane elements. This might be possible by expanding upon the dataframe property and turning it into a param.