Add support for the callback in python/cffi

This uses one global event handler function on the C level, as
enforced by CFFI, and caters for registering a per-context event
handeler.  A simple fixture for doing this during tests is also
implemented.

python/tests/conftest.py

@@ -0,0 +1,15 @@
+import pytest
+
+import deltachat
+
+
+@pytest.fixture
+def register_dc_callback(monkeypatch):
+    """Register a callback for a given context.
+
+    This is a function-scoped fixture and the function will be
+    unregisterd automatically on fixture teardown.
+    """
+    def register_dc_callback(ctx, func):
+        monkeypatch.setitem(deltachat._DC_CALLBACK_MAP, ctx, func)
+    return register_dc_callback