Endpoint markers for interact sliders

[kcrisman]

Endpoint markers for the sliders. Several people wanted to see
what the range of the slider was while sliding, since it might be
difficult to tell otherwise (0 to 1, .1 to .9, ... ). I guess the
request would be to make it possible (or even default) to have the
endpoints somehow labeled.

Component: interact

Issue created by migration from https://trac.sagemath.org/ticket/9639