[BUGFIX] RTE: Magic Images are put in root folder

Due to some problems when find the magic folder, currently
all files from the RTE are put in the root directory of the TYPO3
installation.

However, RTE should use the default upload folder of the user
by default.

Resolves: #46605
Releases: 6.1, 6.0
Change-Id: I4f9f6606bccdeefde563d243a2ad1c9311c6ae81
Reviewed-on: https://review.typo3.org/19224
Reviewed-by: Wouter Wolters
Reviewed-by: Felix Nagel
Tested-by: Felix Nagel
Reviewed-by: Tom Ruether
Reviewed-by: Stefan Neufeind
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter