wish: more feature-compatibility with xscreensaver-demo

[RichardNeill]

Summary

There are some really nice features in xscreensaver-demo that are not ported across to mate-screensaver. It would be delightful if we could have some (or all of them).

Desired behaviour

• It would be nice if,as well as "random" (which selects from all the screensavers), we could have "favourites", which selects from only certain of them. For example, some people find the "ants" rather creepy, or might only want to run light-weight savers, such as "drift" or "flame".

• Each of the xscreensavers has a documentation page. These often include fascinating details on the maths or physics that lie behind the screensaver (see e.g. "man klein") and it would be nice if this were exposed, even simply via an "info button" that opened the man page.

• Many of the screensavers are configurable - what source of text, what image-directory, or a whole lot of sliders to control their parameters. (e.g. skyrocket has amazing sounds, but you wouldn't normally enable them)

Issue #5 touches on this, but that's a 9 year old bug.

An alternative idea would be for mate-screensaver to simply defer to xscreensaver-daemon instead, and to make sure that the control panel and menu settings "plays nice" to ensure that exactly one of the daemons runs, and that the right config UI is invoked, including crossreferences with what starts at boot, and the screenlocker and power-management. (This is more reasonable now than in the past, because the xscreensaver-demo UI now uses modern widgets).

Thank you.