Add snapshot to clipboard feature for moonray_gui_v2

[jlanz]

It'd be quite handy if moonray_gui_v2 had a hotkey to snapshot the current image and store it in the system clipboard so it could be pasted into an email/chat/etc..

[saphalpdyl]

Hey @jlanz, I'd like to work on two of the good first issues: #285 & #284. Planning to get to these over the next couple of weeks. Will open PRs as I go. I want to make sure it isn't assigned internally.