app: make "gegl:introspect" an optional operation dependency.

Check at runtime for the operation availability and set the "Show Image
Graph" action active depending on this check.

This goes with discussions to make this operation optional with a
runtime check for the tool `dot`.
See: https://gitlab.gnome.org/GNOME/gegl/-/merge_requests/84

app/actions/debug-actions.c

@@ -98,4 +98,10 @@ void
 debug_actions_update (GimpActionGroup *group,
                       gpointer         data)
 {
+#define SET_SENSITIVE(action,condition) \
+        gimp_action_group_set_action_sensitive (group, action, (condition) != 0)
+
+  SET_SENSITIVE ("debug-show-image-graph", gegl_has_operation ("gegl:introspect"));
+
+#undef SET_SENSITIVE
 }

app/sanity.c

@@ -650,7 +650,6 @@ sanity_check_gegl_ops (void)
     "gegl:hue-chroma",
     "gegl:illusion",
     "gegl:image-gradient",
-    "gegl:introspect",
     "gegl:invert-gamma",
     "gegl:invert-linear",
     "gegl:lens-blur",