SPARQLStore / ConditionBuilder to use strategy pattern for mapping descriptions

The `ConditionBuilder` is one of the [worst offenders][0] in terms of complexity and class dependency. The following PR introduces a `ConditionBuilderStrategyFinder` interface to map a specific description to a condition builder.

[0]: https://scrutinizer-ci.com/g/SemanticMediaWiki/SemanticMediaWiki/code-structure/master/class/SMW%5CSPARQLStore%5CQueryEngine%5CConditionBuilder