Change all generated files to have name starting with underscore

[fingolfin]

... to simplify .gitignore rules, and clean scripts resp/. make clean targets.

I think this only affects the title page and main page scaffolds. But we have to be a bit careful for projects that generate a title page scaffold but not a main page scaffold: they are relying on the filename title.xml in order to include it. Perhaps we can get away with changing the default name to _title.xml, except in this particular case where we generate the title page but not the main page, when we leave it at title.xml. A user who wants it to be _title.xml in this case, too, can simply specify an explicit filename.

Oh, and for the main page, I would suggest the new default to be to _main.xml and not _PackageName.xml