Enable variables/macros in -runrequires

This works in conjunction with changes in bndtools. I'll post that # in a minute.

After basically implementing this and using it, it's quite true that this is almost exactly like the FileRepo .lib support where you can define a set of bundles as a "library". I suppose this allows not just variables, but macros in general, although I only tested it with variables, since that was the use case I was trying to get working.

My team is setting variables in a cnf/ext/filename.bnd file, and then reusing the variables in various .bndrun files. At least they way we are using it, this works better at the moment, then using .lib files. In the future, I see us using .lib files as we vet a collection of bundles.

Also -- a couple of the changes in BndEditModel could be distilled into getter/setters that work for other variables than just -runrequires, however, baby steps...