GDBus: make use of reliable async cancellation

GDBus: make use of reliable async cancellation

Call g_simple_async_result_set_check_cancellable() after all
GSimpleAsyncResult creation in order to take advantage of the new
reliable cancellation feature.

The guarantee of reliable cancellation fixes a bug in dbusmenu (which
was already assuming that cancellation was reliable).  See this bug:
https://bugs.launchpad.net/ubuntu/+source/libdbusmenu/+bug/953562

https://bugzilla.gnome.org/show_bug.cgi?id=672013

Signed-off-by: David Zeuthen <davidz@redhat.com>