modalDialog: Request HEIGHT_FOR_WIDTH

All our modal dialogs are given a fixed width and grow vertically
as necessary. Set the request mode accordingly, so that wrapped
labels are considered correctly during size request, and not only
at allocation time (where they'll either take away from the padding
or even cause the dialog to overflow).

https://bugzilla.gnome.org/show_bug.cgi?id=704015

js/ui/modalDialog.js

@@ -79,6 +79,11 @@ const ModalDialog = new Lang.Class({
 
         this.dialogLayout = new St.BoxLayout({ style_class: 'modal-dialog',
                                                vertical:    true });
+        // modal dialogs are fixed width and grow vertically; set the request
+        // mode accordingly so wrapped labels are handled correctly during
+        // size requests.
+        this.dialogLayout.request_mode = Clutter.RequestMode.HEIGHT_FOR_WIDTH;
+
         if (params.styleClass != null)
             this.dialogLayout.add_style_class_name(params.styleClass);