Use "text-overflow:ellipsis" to prevent overflow in header

If you open a folder with a really long name, then the folder-name displayed in the header will overflow off the screen, and make the app wider from the screen. (which makes it "pannable" in a way that looks broken).

Patch attached to fix this, by giving this header "text-overflow: ellipsis" (and overflow:hidden, which is required for "text-overflow" to work).