Fix multi-monitor support.

Lokaltog/candybar@970366e5c90e750c562ee3906a3d2764584a0483 changes the relative position of the monitor to the screen, `dest.x`, to 0, preventing gtk from moving candybar from the most right screen to the desired monitor choice.

Though is a bit of a weird way of doing it, I'm not quite sure any others that would work.  Why was this change made initially?