player: add ab-loop-count option/property

As requested I guess. It behaves quite similar to the --loop* options.

Not quite happy with the idea that 1) the option is mutated on each
operation (but at least it's consistent with --loop* and doesn't require
more properties), and 2) the ab-loop command will do nothing once all
loop iterations are done. As a concession, the OSD shows something about
"disabled".

Fixes: #7360