Support for weak clients

[fsobolev]

There's currently no support for weak clients in MPV#.

Docs for mpv_create_weak_client:

/**
 * This is the same as mpv_create_client(), but the created mpv_handle is
 * treated as a weak reference. If all mpv_handles referencing a core are
 * weak references, the core is automatically destroyed. (This still goes
 * through normal uninit of course. Effectively, if the last non-weak mpv_handle
 * is destroyed, then the weak mpv_handles receive MPV_EVENT_SHUTDOWN and are
 * asked to terminate as well.)
 *
 * Note if you want to use this like refcounting: you have to be aware that
 * mpv_terminate_destroy() _and_ mpv_destroy() for the last non-weak
 * mpv_handle will block until all weak mpv_handles are destroyed.
 */