SMW\SQLStore\WantedPropertiesCollector

Test coverage: 100%
CRAP: 12

* Move getWantedPropertiesSpecial() into seperate class to improve
test coverage

* I'll needed the nested Settings stuff in here in order to be able
to group settings

* I'll leave the StoreTest::testGetPropertiesSpecial for now but it is
not how we should do testing (bailing out with something
like $GLOBALS['wgDBtype'] == 'mysql') etc.

* Make StatisticsCollector a member of the Collector class

* The special page only has to ask for ->isCached() to see if the
results are cached

Change-Id: I1732f65103e2b941fd7b73ce00ecc2705d850283