Cache messages so that when infoscreen and doctor's office UI are offline, messages are preserved

[mrts]

Problem cases:

• Patient registers when doctor's view is closed, doctor does not see the registration
• Doctor calls the patient in, after that the infoscreen is restarted, when the infoscreen comes online again, the call-in information is lost

etc.

[mrts]

There's a initial concept draft here: ListManagerBean.java.txt