display: Add 'autoconnect mode' setting (Fixes #311)

Instead of a switch that would always mirror new displays and only
control whether or not to show the minimal dialog we now offer four
default actions when a display is dis/connected:
0: Do nothing
1: Show dialog (default)
2: Mirror
3: Extend
2 jobs for !55 with autoconnect-modes in 3 minutes and 38 seconds (queued for 3 seconds)
detached
Status Job ID Name Coverage
  Build
passed #15639
build

00:01:24

 
  Distcheck
passed #15640
distcheck

00:02:13