Added some power and convenience to preferred content expressions.

Before, "standard" was a special case. Now it's a first-class keyword, so you can do things like "standard or present" to use the standard preferred content expression, modified to also want any file that happens to be present.

Also added a way to write your own reusable preferred content expressions, tied to groups. To make a repository use them, set its preferred content to "groupwanted". Of course, "groupwanted" is also a first-class keyword, so "not groupwanted" or something can also be done.

While I was at it, I made vicfg show the built-in standard preferred content expressions, for reference. This little IDE should be pretty self-explanatory, I hope.

So, preferred content is almost its own little programming language now. Except I was careful to not allow recursion.