[FOLLOWUP][TASK] Remove hard coded extension paths

Remove dependency to t3skin in PageRenderer by adding
clear.gif to EXT:backend. This allows the pagerendeer
to only check for EXT:backend and not to t3skin, which
is not set up on travis.

Resolves: #72525
Releases: master, 7.6
Change-Id: I22202623ba7707d04fc4c71bc389ed1c48384335
Reviewed-on: https://review.typo3.org/45746


Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>