TYPO3 Core

This belongs to the element browser using temporary filemounts added with
options.folderTree.altElementBrowserMountPoints

The current situation has some bugs

• the whole tree is set to "readonly" so user has no possibility to upload files to his regular filemount
• mounts have wrong icon (ftp_user-icon) because the filemounts are initialized with wrong parameter

• remove readonly-flag
• adding a new mount-type "readonly" for temporary filemounts
• add a new icon for readonly-filemount
• use mount type while render subfolders and use readonly-icon for subfolders of temporary mount (see screenshot)
• in element browser determine the current mount and remove upload possibility if mount is readonly

The patch works for all element browsers (normal, RTE, DAM)

Thanks Stan as this patch is created by teamwork of Stan and me.
(issue imported from #M9320)