Add /executable command

This is used to set the openers for various commands.
So far for /avatar and /urlopen.

src/ui/ui.h

@@ -319,7 +319,7 @@ void cons_winpos_setting(void);
 void cons_color_setting(void);
 void cons_os_setting(void);
 void cons_correction_setting(void);
-void cons_avatar_setting(void);
+void cons_executable_setting(void);
 void cons_slashguard_setting(void);
 void cons_show_contact_online(PContact contact, Resource *resource, GDateTime *last_activity);
 void cons_show_contact_offline(PContact contact, char *resource, char *status);