Allow use of domain rules that aren't installed into /usr

Currently Tracker domain rules must be installed inside Tracker's data directory (usually /usr/share/tracker). This is limiting as it means only system packages can add them. A program installed into /opt is unable to use a custom domain, for example. Since Tracker is implemented as a system of daemons, it's not particularly straight forward to work around this by setting XDG_DATA_DIRS= to point somewhere non-standard either.

This patch removes this restriction in a simple way: it allows users to pass a full path to the domain rule, rather than just the base name.