Bookmarking: don't display all folders in the drop down list

Currently, we display all the folders in which a document can be bookmarked in the drop down list, as a flat list. And, when a folder is selected, we display a tree below the list, that shows the possible sub-folders (which are also in the drop down list) and the documents already bookmarked in the tree structure. Mixing a flat list and a structured list on the same screen makes it a bit confusing and difficult to understand. I suggest we only display bookmark folder root in the drop down list and the subfolders in the tree. This would be the same behavior as the Publish tab.

Note: maybe this is fixed now in 5.8, with select2?