macOS: add graceful power-down quit policy

[synrest]

Summary

• Add a “Running VMs on quit” setting with “Save State (Default)” and “Request Power Down” options.
• When requested, resume paused non-suspended VMs, ask all running VMs to shut down gracefully, and wait for every candidate before terminating UTM.
• Cancel application termination and show a visible error when a shutdown request fails or the VMs do not stop within 45 seconds.
• Isolate repeated Quit attempts so delayed completion from an earlier attempt cannot affect a later attempt.

Save State remains the default behavior.

Related to #5912.

Testing

Tested by a human on an Intel Mac running macOS Sequoia with QEMU x86_64 Debian 13 guests. Apple Virtualization was not runtime-tested.

Validated:

• A single running guest shut down cleanly before UTM exited.
• A paused, non-suspended guest was resumed and shut down cleanly.
• Two simultaneous guests both stopped before UTM exited.
• PostgreSQL 17 logs confirmed clean shutdown and clean subsequent startup.
• A guest-side systemd power-key inhibitor exercised the 45-second timeout, visible error, and cancelled termination.
• A repeated Quit while inhibited waited for a fresh timeout and produced exactly one new error.
• Removing the inhibitor allowed a subsequent Quit to succeed.
• The final Settings picker wording and native layout were visually verified.

The author acknowledges that this change has been tested and/or reviewed by a human in accordance with UTM’s AI contribution guidelines.