Gallery2: Front-end attachment reform

Gallery2: Front-end attachment reform

Finally, I understood that the original idea of attaching front-ends
directly from gallery.cf is fallacious. It makes things unnecessarily
complicated, especially if you want to use multiple front-ends.

Now, the only place where we care of the web front-end is gallery.cgi
itself.

Necessarily, we no longer insist that all configuration variables we set
are previously defined. Also, requested thumbnail sizes are now explicitly
mentioned in gallery.cf, because we cannot infer them from the front-ends
(which did not work properly anyway).