Add preference to set locale in standalone

[stakats]

Can use Frank's preference pane icon and ultimately make it one big language setting pane.

Also, we need a new trac component for "standalone."

[dstillman]

We need to add a visible pref for this anyway, but technically this is an issue for all hidden prefs, no? So we probably need a "Config Editor..." option in the Advanced pane of the prefs, like Thunderbird has.

[fbennett]

Just a note that multilingual has grown an embedded UI switcher, in xpcom/multilingual/ui.js. Some of the code there could be used for a separate bibliography preference dropdown, with a tick-box (suggested by Simon) to override the locale in styles that set default-locale.