[TASK] Remove Code Duplication in tslib_content_Media

For every parameter the same code for overlaying Flexform value
over the value given in TypoScript is done.

Furthermore for every file the same url-processing is done.

Remove these code duplication by extracting these things to
external functions for a better readability and maintainability.

Change-Id: I2a614cc1d60f28c3ef4189e74fa3f4dea91fa85f
Resolves: #35440
Releases: 6.0, 4.7
Reviewed-on: http://review.typo3.org/10219
Tested-by: Anja Leichsenring
Reviewed-by: Oliver Klee
Reviewed-by: Anja Leichsenring
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer