libsysprof-analyze: move to internal types for documents

We want to have an object hierarchy, but we don't want to expose our class
or other internal details to the public API/ABI. This uses the old form of
class definition so we can maintain that.

An alternative would be to do what GDK does for internal types, should we
find that easier to maintain going forward.

src/libsysprof-analyze/sysprof-document-frame-private.h

@@ -26,6 +26,14 @@
 
 G_BEGIN_DECLS
 
+struct _SysprofDocumentFrame
+{
+  GObject                    parent_instance;
+  GMappedFile               *mapped_file;
+  const SysprofCaptureFrame *frame;
+  guint                      needs_swap : 1;
+};
+
 struct _SysprofDocumentFrameClass
 {
   GObjectClass parent_class;