monitor-manager-xrandr: Refactor handle_xevent once more

Make the flow a bit clearer, and inline a method only used once.
monitor-manager: Simplify reading the current configuration

Make a wrapper in MetaMonitorManager that handles freeing the existing
configuration for us.
monitor-manager: Put the common hotplug code in a common path as well

So we can reuse it in the KMS backend.
native: Implement monitor hotplug in the native backend

Use gudev to notice when new monitors are hotplugged, and when they are,
update the configuration.