Spotlight: new options for controlling query behaviour

Add three options that allow fine grainted control over the resulting
SPARQL queries:

"sparql results limit = NUMBER", default is unlimited
Useful for limiting the result set for very large systems

"spotlight attributes = STRING", useful for limiting queries to
attributes that are indexed, many are not.

"spotlight expr = BOOLEAN", default: yes, useful for disabling the use
of complex queries that will take too long to complete and hang Tracker.

Signed-off-by: Ralph Boehme <rb@sernet.de>