Merge pull request #15353 (improve xrandrHeads)

When you have a setup consisting of multiple monitors, the default is
that the first monitor detected by xrandr is set to the primary monitor.

However this may not be the monitor you need to be set as primary. In
fact this monitor set to primary may in fact be disconnected.

This has happened for the original submitter of the pull request and it
affected these programs:

 * XMonad: Gets confused with Super + {w,e,r}
 * SDDM: Puts the login screen on the wrong monitor, and does not
         currently duplicate the login screen on all monitors
 * XMobar: Puts the XMobar on the wrong monitor, as it only puts the
           taskbar on the primary monitor

These changes should fix that not only by setting a primary monitor in
xrandrHeads but also make it possible to make a different monitor the
primary one.

The changes are also backwards-compatible.