Make the default mode for newly connected displays configurable

In order to make connecting new displays a little less disruptive I would like to make the default mode for new displays (not: displays that are already "known" and part of a profile) configurable.

The four options "Mirror", "Extend", "Show dialog" and "Do nothing" should be fairly self-explanatory.

The current option "Configure new displays when connected" (that would be replaced) effectively only allows selecting "Mirror" and "Mirror and show the minimal dialog".

Another issue resolved in the process would be #309 (closed)