Handle new Context creation API on the FFI

This adopts the FFI API to use the new Context::new() and
Contest::drop() Rust-style APIs while preserving the semantics of the
existing FFI API.  It introduces a wrapper around the context which
keeps an optional reference to the actual context, changing dc_open()
and dc_close() to create/destroy the actual reference.

This changes the events emitted on closing slightly: they only get
emitted when dc_close() was called while the context was open.  While
dc_close() remains idempotent in that it can still be called safely
multiple times, the close events will only occur if the context is
actually closed.

deltachat-ffi/deltachat.h

@@ -393,8 +393,9 @@ int             dc_set_config                (dc_context_t* context, const char*
  * @memberof dc_context_t
  * @param context The context object as created by dc_context_new(). For querying system values, this can be NULL.
  * @param key The key to query.
- * @return Returns current value of "key", if "key" is unset, the default value is returned.
- *     The returned value must be free()'d, NULL is never returned.
+ * @return Returns current value of "key", if "key" is unset, the default
+ *     value is returned.  The returned value must be free()'d, NULL is never
+ *     returned.  If there is an error an empty string will be returned.
  */
 char*           dc_get_config                (dc_context_t* context, const char* key);