[TASK] Split FLUIDTEMPLATE class to smaller methods

The patch splits the FLUIDTEMPLATE content object to smaller
and more readable code pieces. Additionally, 27 new unit tests
are added for the public API method, checking all important
code areas and documenting current behavior.

Change-Id: I049718a7f00da20534d28353796878310871adf4
Resolves: #43182
Releases: 6.0
Reviewed-on: http://review.typo3.org/16639
Reviewed-by: Helge Funk
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Mattias Nilsson
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn