colo branch: dataset browser column view #503

Currently, the right side displays conf files in the folder selected on the left.
However, I have seen inexperienced users click on another folder randomly after opening a tree and getting confused as to why nothing shows on the right.
Also, it is cumbersome to unfold deep hierarchies because one has to adjust the column width to see the increasingly indented lines.

Maybe we could make a view like the column view in Finder where each tree opens in a new column.