[BUGFIX] T3Editor getPlugins must write to response object

Fix the ajax request target getPlugins to write its response to
the response object and not to the request.

Moreover fix return value of setModeByFile, which is used
only once, but does not need a return value at all.

Resolves: #78802
Releases: master, 7.6
Change-Id: I840de6fe851a3c4e104c4de1ee9bd0e9c4d8f3c6
Reviewed-on: https://review.typo3.org/50781


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>