Install all mozilla-compatible plugins into a common directory

At the moment, nsplugins (mozilla plugins) are installed in a not-uniform way, across different directories and then linked to the plugins directories of other browsers. This makes difficult to remember installing all the plugins in all the right directories (or at least creating the right symlinks).

This purposal is to create a unique directory (say, /usr/lib/nsplugins) and to symlink all of the browser plugins directories to point to that. Packaging should be a lot easier, and we'd limit problems with packages like swfdec-mozilla, dejavu-mozilla, etc.