Add nm-core-types.h, typedefing all of the GObject types in
libnm-core; this is needed so that nm-setting.h can reference
NMConnection in addition to nm-connection.h referencing NMSetting.

Removing the cross-includes from the various headers causes lots of
fallout elsewhere. (In particular, nm-utils.h used to include
nm-connection.h, which included every setting header, so any file that
included nm-utils.h automatically got most of the rest of libnm-core
without needing to pay attention to specifics.) Fix this up by
including nm-core-internal.h from those files that are now missing
includes.