ipc-rules: Add list-outputs function

[soreau]

This can be used by scripts like trailfocus.py, where views need to be divided by outputs. The list can be cobbled together from list_views() and view['output'], but that is a hack, at best.

[ammen99]

I think it might make sense to separate the code from the call for querying a single output to a function and then give the same information with the same structure in both list-outputs and output-info, otherwise lgtm

[soreau]

I think it might make sense to separate the code from the call for querying a single output to a function and then give the same information with the same structure in both list-outputs and output-info, otherwise lgtm

Done.

[ammen99]

Thanks!