authPrompt: let PAM message wrap if longer than entry

Right now the whole authPrompt spreads out if a PAM message
comes in that longer than the entry.

This commit changes it to wrap instead, by forcing the
auth prompt to be a fixed width (slightly bigger than
the entry width was sized to previously).

https://bugzilla.gnome.org/show_bug.cgi?id=705037

js/gdm/authPrompt.js

@@ -96,6 +96,7 @@ const AuthPrompt = new Lang.Class({
         this._entry.grab_key_focus();
 
         this._message = new St.Label({ opacity: 0 });
+        this._message.clutter_text.line_wrap = true;
         this.actor.add(this._message, { x_fill: true });
 
         this._loginHint = new St.Label({ style_class: 'login-dialog-prompt-login-hint-message' });