Support switching worktrees

[hgranthorner]

I use git worktrees quite a bit. It'd be great if gitfourchette supported even just switching work trees via the sidebar. I made a modification to add support for it on my end - if this is a feature you'd like to support in gitfourchette I can make a PR.

[songurov]

I've opened #140 with worktree support: a Worktrees section in the sidebar, plus create, open in a tab, lock/unlock, remove and prune.

@hgranthorner — sorry for the overlap. I'd started on this before I came across your issue and didn't want to sit on finished work, but you asked first and you already have a patch. If yours is closer to what @jorio wants, or if you'd rather send it and have me review instead, I'll happily close mine. I'm also glad to cut #140 down to just the switching part if that's the more welcome size for a first change here.