The next commit will completely rework NMBusManager and replace
NMExportedObject by a new type NMDBusObject.

Originally, NMDBusObject was added along NMExportedObject to ease
the rework and have compilable, intermediate stages of refactoring. Now,
I think the new name is better, because NMDBusObject is very strongly related
to the bus manager and the old name NMExportedObject didn't make that
clear.

I also slighly prefer the name NMDBusObject over NMBusObject, hence
for consistancy, also rename NMBusManager to NMDBusManager.

This commit only renames the file for a nicer diff in the next commit.
It does not actually update the type name in sources. That will be done
later.