user-accounts: Make crop window modal

So it's attached to the panel correctly.

https://bugzilla.gnome.org/show_bug.cgi?id=645700

panels/user-accounts/um-photo-dialog.c

@@ -97,6 +97,7 @@ um_photo_dialog_crop (UmPhotoDialog *um,
                                               _("Select"),
                                               GTK_RESPONSE_ACCEPT,
                                               NULL);
+        gtk_window_set_modal (GTK_WINDOW (dialog), TRUE);
 
         gtk_window_set_icon_name (GTK_WINDOW (dialog), "system-users");