Implement keep on top option in menu

This option allows the user to enable keeping the cheese window at the top. This makes it harder for the user to forget that they have cheese running and it will stay in vision at all times. Pressing F12 or activating it under the hamburger menu enables the feature or disables it if enabled.

I made this because I was using cheese to be able to see my posture while working. See this as a prototype, I have no idea if this is something that other people also like to have. Feel free to close if not, otherwise feedback it welcome.