python: annotate all calls into libnotmuch with types

Add type information to the ctypes._FuncPtr wrappers and
use the wrapper classes instead of c_void_p for pointers
to notmuch_*_t.

This enables the ctypes library to type check parameters
being handed to functions from the notmuch library.

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>