[BUGFIX] RTE Find & replace plugin refers to deprecated function in TYPO3 4.6

this.base() is deprecated in 4.6.
Interim function also raises js error.

Change-Id: I75dbb7e1d9e91a5eb1d76b4fcd64151c115b381f
Fixes: #28818
Releases: 4.6
Reviewed-on: http://review.typo3.org/4208
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland