No horizontal scrolling in tree view

[entepe85]

When working on larger projects on the somewhat constrained screen of a smaller notebook (e.g. MacBook Pro 13" or similar) the tree view quickly becomes to small to display all filenames correctly - at least when the code is still supposed to take up the majority of the screen ;).

Other themes, like the default Atom Dark, support horizontal scrolling in the tree view so I'd very much like to be able to do so with this theme as well.