Rename pkgconfig files
core: Common key computing for notifications

This change adds a common routine for computing the keys
used by notifications. That done, two improvements are added:
1. a 'new-line' character is inserted betwen group name and
key name, it will allow to distinguish the pair ("blabla", "bla")
from the pair ("bla", "blabla");
2. a check is added to produce an error if the key is not valid.

Signed-off-by: José Bollo <jose.bollo@open.eurogiciel.org>