[TASK] Huge performance boost in page tree operations

The main reason for the sluggish page tree was an overly eager cache clearing. The following circumstances had an effect:

* Flux did not know about which type of cache clearing command was being dispatched an content lists need only be rebuilt on full cache clearing.
* Flux is dispatching the cache clearing command to every ConfigurationProvider so operating on the `pages` table caused `tt_content` providers to also trigger, including this one - combined with the first circumstance all FCE definitions would be cleared.
* This Provider rebuilds all content elements when all caches are cleared.

The result was that when operating on pages in the page tree (hiding, showing, moving - any operation that would imply one or more pages' caches being cleared) a log of unnecessary processing was being done. An early return greatly improves this by completely avoiding cache clearing when clearing the cache was not triggered manually.
Merge pull request #51 from NamelessCoder/performance

[TASK] Huge performance boost in page tree operations