Add modResource::filterPathSegment() static method

Implementation of modResource->cleanAlias moved to this new static method which will be accessible without a modResource instance
Add modX->filterPathSegment() method

Provides easy access to modResource::filterPathSegment() | modResource->cleanAlias()
Add filterPathSegment() methods to modX and modResource

Merge remote-tracking branch 'origin/pr/634' into develop

* origin/pr/634:
  Add modX->filterPathSegment() method
  Add modResource::filterPathSegment() static method