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/src/deltachat/__init__.py

@@ -0,0 +1,14 @@
+from deltachat import capi
+
+
+_DC_CALLBACK_MAP = {}
+
+
+@capi.ffi.def_extern()
+def py_dc_callback(ctx, evt, data1, data2):
+    """The global event handler.
+
+    CFFI only allows us to set one global event handler, so this one
+    looks up the correct event handler for the given context.
+    """
+    return _DC_CALLBACK_MAP.get(ctx, lambda *a: 0)