UpdatableAlerts are Alerts whose message texts can be changed after they have
been posted. They are intended to be used to provide ongoing information
about some changing event. For example, the "images received" tracker that
runs during an acquisition is a UpdatableAlert. UpdatableAlerts may be
created via AlertManager.postUpdatableAlert(). This class type is also used
for custom alerts created via AlertManager.postCustomAlert().
Update the text displayed by the Alert. This will affect the text shown
in the main Micro-Manager window for this alert. For simple text alerts
and combining text alerts, this will also update the text in the alert in
the Messages window.
Note that this method is only appropriate for alerts that show the
current status of something (like the acquisition status alert). Usually
you should use a combining text alert rather than update this text