libnm: add NetworkManager.h, disallow including individual headers

libnm: add NetworkManager.h, disallow including individual headers

Add NetworkManager.h, which includes all of the other NM header, and
require all external users of libnm to use that rather than the
individual headers.

(An exception is made for nm-dbus-interface.h,
nm-vpn-dbus-interface.h, and nm-version.h, which can be included
separately.)