Rename "Check Out" to "Switch" in branch UI

Git 2.23 added a new "git switch" command for switching branches: https://github.blog/2019-08-16-highlights-from-git-2-23/#experimental-alternatives-for-git-checkout. This new name is much clearer in explaining what it does than "checkout" ever was.

I use "git switch" at the command line so often now, that when I come to SmartGit I have to spend several seconds scanning the branch context menu for the "Switch" command before I remember it's still called "Check Out".

SmartGit should adopt this new git terminology and use it in the SmartGit UI, as it's a clearer verb for what the command will do, and it matches the future of the git CLI.