Build failure on Continuous

Commit 7f65364e uses g_autolist(GoaObject) but:

• gdbus-codegen does not generate G_DEFINE_AUTOPTR_* gunk for GoaObject (it's an interface)
• you don't control how the GDBus code in gnome-online-accounts is generated, so you can't depend on it as an API because different versions of gdbus-codegen can generate different API

Which ends up breaking the build in Continuous:

../demos/docs-property/docs-property.c: In function ‘main’:
../demos/docs-property/docs-property.c:50:13: error: unknown type name ‘GoaObject_listautoptr’
  g_autolist(GoaObject) accounts = NULL;
             ^
/usr/include/glib-2.0/glib/gmacros.h:1031:48: note: in definition of macro ‘_GLIB_AUTOPTR_LIST_TYPENAME’
 #define _GLIB_AUTOPTR_LIST_TYPENAME(TypeName)  TypeName##_listautoptr
                                                ^
../demos/docs-property/docs-property.c:50:2: note: in expansion of macro ‘g_autolist’
  g_autolist(GoaObject) accounts = NULL;
  ^
../demos/docs-property/docs-property.c:50:2: error: cleanup argument not a function
../demos/docs-property/docs-property.c:50:35: warning: initialization makes integer from pointer without a cast [-Wint-conversion]
  g_autolist(GoaObject) accounts = NULL;
                                   ^
../demos/docs-property/docs-property.c:63:11: warning: assignment makes integer from pointer without a cast [-Wint-conversion]
  accounts = goa_client_get_accounts (client);
           ^
../demos/docs-property/docs-property.c:64:9: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
  for (l = accounts; l != NULL; l = l->next) {
         ^