Improve default icons for folder workflow "add"

The icons used for adding a new document of folder to an existing folder must have a name matching the to-be-executed hippo:query name. If no such icon exists in the project, the default "add document" icon is displayed.

Large projects typically contain many "foldertypes" or hippo:queries, and it is dull and dumb work to copy icons many times just to match the hippo:query name.

I propose a better default behavior of the CMS, such that in many cases, project-specific (copies of) add icons can be avoided: Show the add-folder icon if the foldertype contains the string "folder".

A patch (SVN diff) for this (relative to version 2.26.10) is attached.