Feature: #50039 - Multiple CSS Files in Rich Text Editor

See forge#50039


It is now possible to import more than one CSS file for the Rich Text Editor.

New syntax is:

RTE.default.contentCSS {
        file1 = fileadmin/myStylesheet1.css
        file2 = fileadmin/myStylesheet2.css


The old syntax may still be used. If no CSS files are set, the RTE default CSS file is used as before.