GDBus: Hide class structures for classes we don't want to be subclassed

GDBus: Hide class structures for classes we don't want to be subclassed

E.g. move these C structures out of public header files and into their
respective C files. Also nuke padding since this is no longer needed.

This leaves only GDBusProxy as an extendable type.

Signed-off-by: David Zeuthen <davidz@redhat.com>