Feature proposal: Screensaver enhancements

[kpfleming]

(I'm happy to submit a fully-tested PR for this, just want to ensure that the features are something that are acceptable first).

• Enable screensaver independently for A and B outputs
• Enable screensaver only when an output is not the active output
• Disable screensaver after a maximum amount of time

[hrvach]

Yes, all of those are good ideas and I'll be more than happy to merge them! I'm very grateful for any help and PRs. :)