client API: simplify (?) property change notification generation

Property change notification works by having the mpv core wake up all
clients observing a property when the property potentially changes. The
clients then read the property's value, and determine if there was an
actual change. (The latter part depends what the property returned for
the previous change notification, so it depends on the client, and
cannot be generated by the core itself.)

Until now, reading the property value was done in a pseudo-async way by
queuing a callback back to the core, running it there, and then waking
up the client thread again. I cannot comprehend why this was done in
such a complicated, fragile way. Maybe it's a leftover from times when
client.c had to do this (in short, because properties could access
vo_opengl, which has thread-local state).

One past idea was to make the implementation of true async properties
easier (for which you would need such a state machine anyway). But they
don't exist yet, and I doubt the current mess would be really helpful
when actually implementing them.

Simplify this, and run the update in the client's thread directly. In
addition to the fundamental change, many roundabout things can be
removed as a consequence.

Unfortunately, I noticed that lock order issues force you to release
ctx->lock before doing so, which makes things more complex due to
possible concurrent mpv_unobserve_property() calls. Solve this by
removing properties lazily, which means you may have to do multiple
mpv_wait_event() calls before the property entry is actually destroyed.
This should not matter in practice, and does not affect the semantics.
It could also cause "leaks" by observing/unobserving properties in a
loop, without ever calling mpv_wait_event(). Just don't do this, duh.

(I considered making this dependent on whether the previous
mpv_wait_event() call returned the property being removed, but a
separate code path seemed too complicated. I also considered copying the
name and property data when returning a MPV_EVENT_PROPERTY_CHANGE, but
actually this doesn't solve the problem of update_prop() being
interrupted by mpv_unobserve_property(); there are ways around it, but I
just said no.)

This was made using the cowboy coding software engineering methodology.
If you find any bugs, keep them yourself.

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