client: Warn on _create_input_context() if cancellable != NULL

This should never happen so don't try to cancel the operation and
continue silently ahead.
client: Queue events while the IBus context isn't ready

We may lose events that ought to be processed while the IBus context
isn't ready or if the connection to IBus isn't fully established yet.

To avoid that, enqueue events to be processed later when the IBus
context creation finishes.