Jekyll treeview should be horizontally scrollable and/or have a button to hide

[becdetat]

Because of the way the treeview expands to accomodate the file names, when I open a posts folder with long names it takes up over half the screen. It should be fixed to say 1/5 of the width, with a horizontal scroll, or have a button to minimise the tree (I remember @JakeGinnivan had a minimise button in an early version), or ideally both.

The width (or hide status) should also carry through to a new editor if opening from the site.