[TASK] Define public API for TYPO3CR

This patch provides @api flags for all methods belonging to
the public API (userland) of TYPO3CR. Along the way
interfaces and doc comments have been cleaned up and
one missing method in ProxyNode was implemented.

This change should not contain any functional changes.

Change-Id: If51243d116ea1c3d22440708c2a4033df759fe45