swaybg: one instance for all outputs

This makes it so there will only be one swaybg instance running
instead of one per output. swaybg's cli has been changed to a xrandr
like interface, where you select an output and then change properties
for that output and then select another output and repeat. This also
makes it so swaybg is only killed and respawned when a background
changes or when reloading.