cssnode: Convert name + id from interned string to GQuark

cssnode: Convert name + id from interned string to GQuark

The reason for this is simply that I want to get hash functions that
have their values close together, so they can fit in a smaller range
(the goal here is 12 bits). By using GQuark, we get consecutive numbers
starting with 1 (and applications have <1000 quarks usually), whereas
interned strings can be all over the place.

As a side effect we also save 64 bytes per declaration.