"Screenshot" button is poorly integrated

[JayFoxRox]

I think we should remove the "Screenshot" button - most operating systems have a feature for this built-in anyway (example: Alt+PrtSc on Windows puts screenshot of active Window in clipboard).

Having support for taking screenshots can potentially be useful for telemetry or bug reports, but this would need better integration.

Issues with the current implementation:

• A standalone screenshot feature feels unnecessary to me.
• Button is always shown, even when XQEMU is not even running.
• There's no confirmation that a screenshot was taken.
• The user does not know where the file is saved.
• Only a single screenshot file (screenshot.ppm) is allowed.
• PPM is not widely supported, especially in browsers, so screenshots can not be shared too easily.

I think we should remove the button for now to avoid all of these issues.