winemac: Make macdrv_process_text_input() asynchronous and process internal events while awaiting its result.

It had been using the synchronous OnMainThread() to submit its work to the
Cocoa thread, but only queries are processed while OnMainThread() waits for the
work to complete.  This led to QUERY_IME_CHAR_RECT queries being processed out
of order relative to IM_SET_TEXT events, making the character range out of
bounds with respect to the composition string.
Signed-off-by: Ken Thomases <ken@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>