rename "semi-private" methods/properties that other parts of the codebase hook into #1015

like Graphic._plot_area, etc, I suggest something like ipython's convention with single under trailing and leading: https://ipython.readthedocs.io/en/stable/config/integrating.html

Will make a full list of methods and properties where this would be useful, I started using this convention in NDWidget

Graphic._plot_area_
Graphic._set_plot_area_
Graphic._prepare_del_
BufferManager._gpu_buffer_
Graphic._address_