command: add a property to indicate core pause state

Currently this is (probably) equivalent to "paused-for-cache", but the
latter is a bit special, while this new property is a bit more general.
One case where they might actually be different is dvdnav menus, but I
haven't checked.

Also add property change notifications for these two properties.
command: add property to indicate when pausing due to --keep-open

This property is set to "yes" if playback was paused due to --keep-open.

The change notification might not always be perfect; maybe that should
be improved.
client API: remove mpv_event_pause_reason

And slightly adjust the semantics of MPV_EVENT_PAUSE/MPV_EVENT_UNPAUSE.

The real pause state can now be queried with the "core-idle" property,
the user pause state with the "pause" property, whether the player is
paused due to cache with "paused-for-cache", and the keep open event can
be guessed with the "eof-reached" property.