extensions-tool: Add common option to silence errors

Error reporting is useful when used interactively, but often undesirable
when used in scripts. Account for that with a common --quiet option,
which is more convenient than redirection stderr to /dev/null.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/2391

subprojects/extensions-tool/src/common.h

@@ -45,6 +45,8 @@ typedef enum {
   DISPLAY_DETAILED
 } DisplayFormat;
 
+GOptionGroup *get_option_group (void);
+
 void show_help (GOptionContext *context,
                 const char     *message);