Gallery / files: bring back "create folder" functionality

The "add image" and "add file" shortcuts will now always upload to the root of either the /content/gallery or /content/assets folder.

By contrast, the "New document" shortcut creates a new document either under a default root (/content/articles) or in the currently highlighted folder within the tree. That behavior would be perfect for the add image and add file shortcuts as well.

Than it makes sense again to be able to create folders.

This issue mainly means that both shortcuts need to be extended with the "new document" behavior, and then the "New Folder" and "Delete Folder" functionality that had been disabled for /gallery and /assets should be enabled again.