js-info disabling sidebar for narrow windows (experimental)

[Per Bothner]

The js-info reader doesn't attempt to handle narrow windows/screens.
So I implemented something a bit more flexible.

For a rough prototype, see: http://per.bothner.com/tmp/DomTerm-txjs/

Note there is a "Hide sidebar" button (with crappy styling) on the lower left.
It does the obvious.

Furthermore, if the window is "narrow" then the sidebar is "transient":
Tt is automatically hidden when you click on a page link (in the sidebar).
There are also styling and size changes, most notably that the
sidebar overlays the main-bar, which is always full-width.

A goal was to automatically switch to "narrow" mode on smartphones,
that isn't working yet.  (Smartphone browsers tend to do magic
things with "viewport" and zooming, and dealing with that needs
some more research and experimentation.)

There are other issues.  Clicking on the cross-links in the mainbar
doesn't currently work when the sidebar is hidden.

Feedback welcome.
--
        --Per Bothner
per@bothner.com   http://per.bothner.com/