Add feature to disable VM screenshots

[gabefair]

This setting is found by hovering over the VM's preview window in the VM library. It is a per VM setting so the user can decide which VM's can have the preview screenshots. This hover over menu also allows the user to save the screenshot, which can be helpful for debugging. This feature does not need the VM to be off to implement.

[insidegui]

Thank you for the contribution @gabefair!

I'm not ready to accept this pull request as-is though, since it introduces a change that will conflict with future enhancements that I'm already working on.

I would accept a pull request that simply adds a new global on/off toggle for virtual machine screenshots (accessible from the app's settings window), as that would remain useful even after I ship the aforementioned changes.

I'm sorry I can't merge this, as it seems like you did a great job. I'm working on introducing contribution guidelines that will prevent this sort of thing from happening in the future.

[gabefair]

No worries, I will refactor and update the PR once your changes are mainlined.