Backport to 7.9: Allow adding custom fields to content creation dialogs

For a project, we want to add extra fields and logic to the dialogs for creating a new folder and uploading a new resource. We currently need to fork some classes and HTML templates to achieve this. I tried to make the changes minimally invasive and discussed them with Unico. I'm attaching a diff relative to the 7.9 CMS branch on 12/01/2015 (POM version 2.26.12-SNAPSHOT), identifying the relevant changes.