Electron app improvements

[mattgodbolt]

Reported on Stardot: the standalone electron app is missing the rewind feature. We should audit the web version and look for any other missing functionality too.

[mattgodbolt]

Additional feedback (which may be worth filing as separate issues):

The rewind feature seems to not work in the downloadable version, only on the web. I downloaded it yesterday so it's the latest version. It's also not clear what the keyboard shortcuts are when the debugger is open, and it took me ages to work out G resumes it (as the play button isn't visible in the standalone version).

The ability to change the emulation speed on the fly would be nice too - I know Ctrl+Insert is max speed but sometimes you just want it a bit faster.