app, etc: do not hide --show-debug-menu.

This is a useful debugging function for developers. It is enough to hide
the menu by default on stable releases, I don't think we also need to
hide the CLI option. Developers don't know all these options by heart,
so we need to make them reasonably discoverable!

app/main.c

@@ -297,7 +297,7 @@ static const GOptionEntry main_entries[] =
     N_("Show a preferences page with experimental features"), NULL
   },
   {
-    "show-debug-menu", 0, G_OPTION_FLAG_HIDDEN,
+    "show-debug-menu", 0, 0,
     G_OPTION_ARG_NONE, &show_debug_menu,
     N_("Show an image submenu with debug actions"), NULL
   },

etc/bash_completion.in

@@ -34,6 +34,7 @@ __gimp()
         "--pdb-compat-mode"
         "--session"
         "--show-playground"
+        "--show-debug-menu"
         "--stack-trace-mode"
         "--system-gimprc"
         "--verbose"