client API: avoid returning stale value on property notifications

This could happen if a property was flagged as changed, then updated,
then flagged again, but gen_property_change_event() was called before
the value was updated a second time. Then the function simply returned
the old value, and would later trigger a new change event again.

This was considered acceptable before, since property notifications are
asynchronous anyway (so they may always be "outdated", it just mattered
whether the most recent value was eventually delivered).

But consider ordering with events. It seems desirable that specific
important events (e.g. MPV_EVENT_START_FILE) should not be followed by
property updates that happened before it, because that would make
application logic really a mess, and property notification near-useless
in certain cases.

Avoid this by never returning a value if it was marked changed, but not
updated yet.

Unfortunately, this could lead to clients never receiving a value (or
receiving it with a high random delay), if they're too slow to read it
(or the property simply updates too often). Note that this is done for
_all_ property notifications, not just returned events. Hopefully not a
problem in practice. If it turns out to be one, this mechanism could be
restricted to actually returned events, for which this really matters.

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