NMActiveConnection:connection was an object path rather than an
NMRemoteConnection because in the past the NMObject property system
wasn't capable of handling NMRemoteConnection-valued properties.
NMActiveConnection:master was an object path rather than an NMDevice
entirely by accident. Fix both.

NMActiveConnection:specific-object still has to be an object path
because we don't know ahead of time what object type it is, and
NMObject can't deal with that.

Add nm-types.h defining all the type structs, to avoid future include
loops. Clean up the includes in all of the installed headers.

Make NMRemoteSettings internal and have NMClient wrap all of its APIs,
just like it does with NMManager.

Rename NmCli.system_connections to connections, since the "user
connections" have been gone forever.

Clone NMClient as NMManager, and make NMClient just be a wrapper
around that new class.

Add a single function to check if NM is running and set a GError if
not, then use it as appropriate.

Don't bother to check if NM is running in get_*() functions if
nm_running_changed_cb() would have reset the field anyway (and fix
that up to reset a few more fields).