[BUGFIX] EXT:form - themeName can not be overwritten by integrator

It is possible to set a new themeName for a specific form but it is not
possible to set it for the whole plugin via

plugin.tx_form.settings.registeredElements.FORM.themeName = Mytheme

Resolves: #70368
Releases: master
Change-Id: I76e7584dd9b333c56c0dc4c05a7d5e78b0a7c225
Reviewed-on: http://review.typo3.org/43805


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>