Allow to horizontally resize the sidebar (Set section)

[keeganedwin]

Is your feature request related to a problem? Please describe. The plugin right now seems to be able to handle nested folders well, but to maybe make up for some space around with a swamped nested set side-bar.

(Edit) In-addition: Resizable sidebar that remembers it’s size between sessions.

Describe the solution you'd like Allow to horizontally resize the side-bar (Set section).