[FEATURE] Create interface and enable custom form_postProcessors

[FEATURE] Create interface and enable custom form_postProcessors

PostProcessors now need to implement the
tx_form_System_Postprocessor_Interface.
All classes implementing this interface can be loaded as a
postProcessor by passing its class-name by typoscript.

Before loading the class in the namespace 'tx_form_System_Postprocessor_'
tx_form_System_Postprocessor first tries to load the class passed by
typoscript directly.

Change-Id: I3bea8ce8287a6713f483d7e019cb5f47cdc68cb8
Resolves: #32701
Releases: 6.0
Reviewed-on: http://review.typo3.org/9006
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog