output-management: add a way to trigger a rescan

[emersion]

According to a recent discussion on dri-devel, userspace should provide a way to force a full reprobe of the DRM connectors:

User opens the screen configuration tool -> usually at that point the
tool/compositor force a full reprobe, which then often triggers the
automatic reconfiguring. E.g. on one laptop I have here when I plug in
random shit projectors at conferences nothing happens, until I run
xrandr, which triggers the full reprobe, which then makes the kernel
realize something change, sending and uevent, which starts the
automatic reconfigure machinery.

This is really not great, but the kernel folks say there's no way around that.

[emersion]

Users can trigger a re-scan manually with:

echo "detect" > /sys/class/drm/<card name>/status

[emersion]

wlr-protocols has migrated to gitlab.freedesktop.org. This issue has been moved to:

https://gitlab.freedesktop.org/wlroots/wlr-protocols/-/issues/47