Files: Ability to resize the sidebar

[XeZrunner]

Is your feature request related to a problem? Please describe.
It isn't related to a problem.

Describe the solution you'd like
When hovering between the sidebar and content edge, the cursor should change into the horizontal resizing cursor and let us change the sidebar width. The application should also save this width so we don't have to keep adjusting the sidebar width every time.

Describe alternatives you've considered
Increase the default sidebar width, to match Fluent Design / Windows guidelines.

Additional context
Here's a quick mockup done in Paint.

[chuchanpr]

Add option to collapse the side bar, for cleaner look

[yaichenbaum]

This has been implemented with #1512.