Allow panel area to resize.

[bungeman]

When there are many axes the "Delta Sets" view can get quite wide, and
other panels can also get a bit crowded. Allow the user to resize the
panel area.

Note that this is more a request for this feature with a simple possible implementation. This change was made in a local fork to make it a bit easier to look at fonts which make the panels wide.

[Lorp]

Thanks for this. As you suggest, the resize CSS property is a really simple way of doing it. However it seems rather fiddly to control, sometimes impossible to control, in Safari. I’ll likely add it soon despite that.