client API: deprecate some events

Following the discussion in #1253.

The events won't be removed for a while, though. (Or maybe never, unless
we run out of bits for the uint64_t event mask.)

This is not a real change (the events still work, and the alternative
mechanisms were established a few API revisions earlier), but for the
sake of notifying API users, update DOCS/client-api-changes.rst.
command: fix option-flags property

The sub-path wasn't adjusted, and it worked only in some situations.
demux_mkv: fix undefined shifts

Found by clang sanitizer. Casting unsigned integers to signed integers
with same size has implementation defined behavior (it's even allowed to
crash), but it seems reasonable to expect that reasonable
implementations do a complement of 2 "conversion".
command: silence a warning on win32

Same goal as a change in the #1255 PR, but IMO slightly less
ifdefferish.
win32: silence some warnings

Signed-off-by: wm4 <wm4@nowhere>