wip: replace xinerama with xrandr

[N-R-K]

xrandr is supposedly the better replacement for xinerama. On my system, no packages (other than scrot) use xinerama while there's a bunch using xrandr, so it's likely a more common dependency which is probably already installed too.

Alternatively, instead of replacing it we could offer a build time config switch allowing the user to select between xrandr and xinerama.

[N-R-K]

Anyone with multi monitor wants to try this out? The patch should be functional at it's current state. Though documentation and CI needs updating before merging - I'll do it later once the patch is confirmed to be good.

ping: @daltomi @zevweiss