Disconnect/reconnect parameters on suspend/resume

Disconnect parameters to update_plot on suspend_update, reconnects on
resume_update (if _get_auto_update_plot() returns True). Also changes
_get_auto_update_plot() to check for suspension, as several internal
functions rely on it. Because of this, it is important that
resume_update decrements the counter before it checks
_get_auto_update_plot.