Mios datetime handling

Automatically convert any MiOS Epoch-style date into what's needed for openHAB, if the corresponding bound Item is of type DateTime.

MiOS uses Epoch-style (numeric) dates in a variety of the shipped UPnP StateVariable declarations, as well as many of the custom ones defined by Developers. This code is more general, and replaces the static-list of UPnP Service/StateVariables previously used (Long -> DateTime conversion)

Obsoleted the `DatetimeVariables.properties` file, since it's no longer needed, as well as cleaned up the comment formatting in code that I was touching.