[BUGFIX] RTE: Consistent naming for menutabs at "Insert image"

The various kinds of an image are named like
image, magic, plain and dragdrop. However their
names in the menudefinition are different.

Fixes: #34026
Releases: 4.7

Change-Id: I3cb32c0afb5c1000fb8ff59721794c97c77b8e22
Reviewed-on: http://review.typo3.org/9065
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland