[FEATURE] Allow adding multiple CSS files in RTE

Allow to add more than one CSS file in the RTE header.

The new syntax is like this:
    RTE.default.contentCSS {
        file1 = fileadmin/myStylesheet1.css
        file2 = fileadmin/myStylesheet2.css
    }

If no CSS files are set, the RTE default CSS file is still added.

Releases: master
Resolves: #50039
Change-Id: I4b8545982a59382ca4e01abc4440f0b94693ccdc
Reviewed-on: http://review.typo3.org/24523
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>