Make a number of window focus related effects optional

I have added these console variables that apply when the window loses focus.

• showfocuslost On by default; toggles showing the "FOCUS LOST" overlay.
• playmusicifunfocused Off by defautl; toggles pausing music.
• playsoundsifunfocused Off by default; as above. This one is actually new behavior by default.
• pauseifunfocused On by default; toggles the game pausing in off-line mode.

Taken from context of some folks on Discord. Demo GIF of showfocuslost. https://cdn.discordapp.com/attachments/270211185666686978/554754099904774157/notinfocus.gif