Provide more featureful "Shutdown" dialog for prompt to shutdown

[cassidyjames]

In the power plug, we have an option to prompt to shutdown on power button press. However, it might be more useful to show a dialog that also includes more power-related actions, like restarting and suspending, and arguably even logging out. This would be more convenient on hardware like the Pinebook Pro where you could just tap the power button and then have your choice of things to do depending on your current context.

However, we don't want to always show these options, just in certain externally-triggered contexts. So I imagine this would be a new dialog used specifically for when the shutdown prompt is triggered externally.

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