Rework monitors processing for desktop manager.

[LStranger]

It should not enumerate them but remember plugs instead and associate config with them. When monitor is getting plugged off then plugged on, it will get remembered config back.

This change is made instead of #3 to handle it more correctly. Some fixes from #3 should be pulled separately though.

[wb9688]

It seems to be broken now: when I add a monitor, it doesn't come up there, while when I remove a monitor, it gets stuck in a loop in renumber_monitors.