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!

(cherry picked from commit 083360fac1)

app/main.c

@@ -310,7 +310,7 @@ 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"