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/_build.py

@@ -28,6 +28,13 @@ def ffibuilder():
                           output_file=dst_fp.name,
                           macros=[('PY_CFFI', '1')])
             builder.cdef(dst_fp.read())
+    builder.cdef("""
+        extern "Python" uintptr_t py_dc_callback(
+            dc_context_t* context,
+            int event,
+            uintptr_t data1,
+            uintptr_t data2);
+    """)
     return builder