Remove the context reference from Message struct

The Message struct had a reference to the context which made a few
APIs a little easier.  However it has surprising consequences a long
way down the line as shown in #335: it means any object which has such
a reference needs to keep open a lock if we want to do this refactor
of no longer having a "closed" Context struct on the Rust API (which
has many benefits which will simply that Context struct and is more
the Rust way - RAII etc).

By refactoring away the context reference on the rust API as done in
here however, we push this behaviour of how these references are
handled back to the C-API pointer behaviour: that is unsafe but just
works in a C-like way.  The resulting complexity in the FFI layer is
also notably less than in the #335 alternative.

As a consequence all APIs which require the context, now explicitly
need to get the context passed in as an argument.  It looks like this
is certainly no downside and maybe even beneficial for further API
refactors.

For this strategy to work out the same should be done to
dc_chatlist_t, dc_chat_t and dc_contact_t.  But this working for
dc_msg_t give a reasonable confidence that this is a good approach.

src/securejoin.rs

@@ -274,7 +274,7 @@ fn send_handshake_msg(
     fingerprint: Option<String>,
     grpid: impl AsRef<str>,
 ) {
-    let mut msg = unsafe { dc_msg_new_untyped(context) };
+    let mut msg = dc_msg_new_untyped();
     msg.type_0 = Viewtype::Text;
     msg.text = Some(format!("Secure-Join: {}", step));
     msg.hidden = true;