manpage: remove duplicated misplaced description of a command
command: add a property that returns a list of all properties

Also remove the undocumented Lua mp.property_list() function.
TOOLS: add test script for property change notifications
client API: minor optimizations for property notification

Internally, there are two mechanisms which can trigger property
notification as used with "observed" properties in the client API.

The first mechanism associates events with a group of properties that
are potentially changed by a certain event. mp_event_property_change[]
declares these associations, and maps each event to a set of strings.
When an event happens, the set of strings is matched against the list of
observed properties of each client. Make this more efficient by
comparing bitsets of events instead. This way, only a bit-wise "and" is
needed for each observed property. Even better, we can completely skip
clients which have no observed properties that match.

The second mechanism just updates individual properties explicitly by
name. Optimize this by using the property index instead. It would be
nice if we could reuse the first mechanism for the second one, but
there are too many properties to fit into a 64 bit mask.

(Though the limit on 64 events might get us into trouble later...)