Split off implementation details of MdModuleNotification

Split off implementation details of MdModuleNotification

For better readability and testing, moved the MdModuleNotification and
MdModulesNotifier into separate files, hiding away the implementation
details of the complex general callback-infrastructure and exposing more
clearly the callbacks that MdModules can sign up to.

Moved tests into correct directory.

Moved life-line explaining the notification machinery in MDModules from
MdModulesNotification to MdModuleNotifier, where it is better suited.

Change-Id: I0a73f99e450873903a888bcc04228737d0b814f3