Introduce CallsUiCallData to feed CuiDisplay

We need a proxy object because implementing the CuiCall interface in the
CallsCall base class has it's own set of problems (f.e. the "state" property
would have a type collision CallsCallState vs CuiCallState).

src/meson.build

@@ -116,6 +116,7 @@ calls_sources = files(['calls-message-source.c', 'calls-message-source.h',
                        'calls-settings.c', 'calls-settings.h',
                        'calls-secret-store.c', 'calls-secret-store.h',
                        'calls-network-watch.c', 'calls-network-watch.h',
+                       'calls-ui-call-data.c', 'calls-ui-call-data.h',
                       ]) + calls_generated_sources
 
 calls_config_data = config_data