Previously, we were getting screen info every time we tried to move the
window. To improve the situation, information about the available
screens is initialised once when dunst starts and further behaviour
depends on the compile-time options used.
If Xrandr is enabled, screen information is updated when an
XRRScreenChangeNotify event is received, meaning only when the screen
layout changes.
If Xinerama is enabled, screen information is only updated on startup.
This behaviour might be changed later.
If none are enabled, then dunst assumes only one screen and ignores all
multi-monitor options.
Since we are adding Xrandr support to allow for automatic dpi detection,
split screen handling code into a new file as part of the effort to
simplify x.c
