mpl widgets: LassoSelector/RectangleSelector

Hi,

I build a backend for this package in [vaex](http://vaex.astro.rug.nl), so vaex is showing many rows/particles in this matplotlib backend. However, I'd like to select things using the lasso or rectangular selector, the rectangular works ok, it draws sth on the screen (see also screenshot). To enabled it I used a [ipywidget togglebuttons](https://github.com/maartenbreddels/vaex/blob/master/vaex/notebook/ipympl.py), but it doesn't play nice with the other buttons (for instance, how would I switch between zoom rectangle and the rectangleselector?). And the lasso selector doesn't draw anything on the screen. Would be really nice to have this, it would be the first time you could do a lasso selection in the notebook to do a freehand selection for ~1 billion rows.


Review vaex/notebook/ipympl.py to understand how the ipywidget toggle buttons currently switch between tools. Then inspect the notebook widget integration for selector activation and drawing behavior. Done means zoom, rectangle selection, and lasso selection can be switched reliably, with the lasso visibly rendering and supporting notebook selection.

Written by the indexing model from the issue text.