Feature request: add an option to hide header/title bar in NbAgg/WebAgg

*This is a cross-post of https://github.com/matplotlib/matplotlib/issues/11986 as requested in that issue - this issue also applies if I use jupyter-matplotlib*

I am making use of the NbAgg backend to implement data viewers for the [Glue](http://glueviz.org/) application in Jupyter notebook. However, one thing that I've found is that the title/header bar is unnecessary in a lot of (my use) cases and there is also a lot of wasted space between the title/header bar and the plot itself:

This is even more the case when opening multiple plots in a single notebook.

Would it be possible have a way to turn off the header/title bar and get a tighter layout? (or is there already a way to do this?). In my mind it would look nicer to simply have:

And for my use case I'm fine with losing access to the 'power' button to freeze the plot.

Start by tracing the NbAgg/WebAgg entry points used by jupyter-matplotlib and compare the current header/title bar with the requested compact layout. Done means an option can disable that bar while retaining the existing plot behavior, with the resulting layout checked in a notebook.

Written by the indexing model from the issue text.