[libcommhistory] Fix exporting and behavior of ClassZeroSMSModel

Class zero events have no ID, so the eventModel logic would ignore all
events after the first one.
[libcommhistory] Improve ClassZeroSMSModel API and export to declarative

The deleteEvent function was not useful, as class zero messages have no
ID (since they are never stored). A clear function was added instead to
clear the model.

Also added a newMessage signal for more convenient usage, especially
from QML.
Merge pull request #47 from special/master

Improve ClassZeroSMSModel API and export to QML