UI to allow long-running commands in Linux without hibernation on blur

We have code in the terminal to halt the Linux VM whenever xterm.js loses focus. We should probably make this something you can opt out of, if you need to leave some long running command going while you switch tabs. We might also want to overlay the terminal with some kind of "paused" or grayed out state when you click away from it, to mimic what a VM does when it's halted.