[qtcontacts-sqlite] Various improvements

Improve support for self contact (pre-populate database with a self
contact, which is (by default) not returned during normal fetches
via filter; don't allow set or remove of self contact id).

Improve support for avatars (allow custom "AvatarMetadata" field
which stores arbitrary string data).

Improve debug support for filters (more verbose debugging output).

Remove application name from database storage location (as this would
cause a per-application contacts database to be created).

Synthesise display labels properly for aggregate contacts.