I’m looking to see if this type of widget is supported in 4.5+ (currently working out of 4.4). If not, I have an idea of how to make one; high-level explanation follows. Thanks!
The idea is to create a Tree-like structured widget in UMG.
To start, I'd make a custom widget that contains a Button (with an image) and a Label within a horizontal box. Next, I would have the blueprint include a boolean (to signify if this item is currently "open" or "closed"), a list of child widgets (branch/tree/leaf items), and a number signifying this leaf item's level. If this item has no children in its list, then the button becomes invisible, otherwise it defaults to the "closed" state and displays the corresponding image on the button. When the button is clicked, the state is now "open" and the button swaps images.
Next, I'd create the master tree view, which would start as a vertical box. This holds a list of leaf items (described previous). Simply show each child in this list with a number of spacer controls before them equal to their individual level. Clicking a branch's button to "closed" hides the corresponding children.
Very simple overview, but that’s the approach I would start taking if I were to make this myself. Cheers!