Later, we should move all such objects. And we should rename
the API to have a unique prefix, like "NMPPlObjIP[4]Address".
This is just a first step that introduces more inconsistencies than it
solves. It will get better afterwards.
Our naming in libnm-platform is bad.
We have NMPlatform, which is a cache of objects. Consequently we have
platform methods like nm_platform_get_link().
We also have various other types that share the NMPlatform prefix, like
NMPlatformIP4Address. For those we have nm_platform_ip4_address_to_string().
"methods" of a type should have the same prefix as the type,
and we should not have types that share the same prefix.
Also, "NMPlatformIP4Address" is a long name, and inconsistent with the
strongly related NMPObjectIP4Address.
Add new files to move and rename parts of the platform API.
