app: add --show-debug-menu command-line option

The debug menu is currently not included in stable versions.

Include the menu unconditionally, but hide it, and its associated
actions, by default in stable versions.  Allow enabling the menu
using a new --show-debug-menu command-line option, in the same vein
as --show-playground.

app/app.h

@@ -48,6 +48,7 @@ void  app_run       (const gchar         *full_prog_name,
                      gboolean             console_messages,
                      gboolean             use_debug_handler,
                      gboolean             show_playground,
+                     gboolean             show_debug_menu,
                      GimpStackTraceMode   stack_trace_mode,
                      GimpPDBCompatMode    pdb_compat_mode,
                      const gchar         *backtrace_file);