ACPI shutdown does not work on headless VMs / needs a display configured

If run a headless Linux, I do not configure a display.
This is from the documentation:

> To setup headless mode, open the virtual machine configuration and [delete any display device](https://docs.getutm.app/settings-qemu/devices/devices/) as well as any serial device that is set to “Built-in Terminal” mode.

https://docs.getutm.app/advanced/headless

However, if I have no display configured, I loose the ability to invoke an ACPI shutdown, because the VMs display seems to be the only place to reach the functionality.

## Proposals
Add a context menu entry, so a user can right-click the VM and power it down:


Make the long click available in the main window:

I have no preference where the button should be, but I would appreciate its existence.

## Relates to
- #4001
- #5912