Lib: don't use g_idle_add, respect thread affinity

After commit 4e85baaf libsignon-glib stopped working when used in a thread on which g_main_context_push_thread_default() had been called: the reason is that g_idle_add()'s callback is always involved on the default GMainContext, and not on the one used by the current thread.

We fix this problem by using g_source directly. Fixes: https://bugs.launchpad.net/bugs/1607799