app: move the code to update scrollbars and rulers to their own files

so they are gone from the "scale" and "scroll" namespaces because they
belong to neither and both. And because the files are way too large.

app/display/gimpdisplayshell-rulers.h

@@ -0,0 +1,25 @@
+/* GIMP - The GNU Image Manipulation Program
+ * Copyright (C) 1995 Spencer Kimball and Peter Mattis
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef __GIMP_DISPLAY_SHELL_RULERS_H__
+#define __GIMP_DISPLAY_SHELL_RULERS_H__
+
+
+void   gimp_display_shell_rulers_update (GimpDisplayShell *shell);
+
+
+#endif  /*  __GIMP_DISPLAY_SHELL_RULERS_H__  */