[TYPO3-team-core-v5] [!!!] Switched from @internal to @api

[Karsten Dambekalns]

Hi.

As per http://forge.typo3.org/issues/show/3883 I removed all @internal 
annotations and added @api for methods in the public API instead.

Fluid should be checked by someone deeper into things - I was not sure 
the annotations are now as they should be.

Everyone: whenever you add a method that should be used "from the 
outside", remember to annotate as @api from now on.

Karsten