Implement PSR-0 standard and proper use of namespaces

As described in this change record (and others), Drupal 8 implements the PSR-0 standard and uses that for auto-loading. Consequently, namespaces are now also used heavily.

We therefore have to adapt our file structure to support this, and use proper namespaces. I have already done the first step and moved all classes and interfaces to separate files in appropriate locations (closely resembling the structure in core modules, as far as I understand that currently). I also renamed them (mostly dropping the suffix) and added matching namespace declarations.

Still to do

• Update references to used core files.
• Add necessary use declarations.
• Add namespaces to classes/interfaces referenced in comments.
• Discuss whether some classes/interfaces should be moved to a different location (and, hence, namespace).

Anything else?