Investigate punting windowed mode indirection

[davidben]

Both the old Xt-based and new XEmbed-based windowed modes involve creating viewer-side stubs for the plugin to attach itself to. At least for the XEmbed one, we should be able to do away with it and embed into the parent directly. Chrome manages this, and everything is now completely synchronous, so there shouldn't be issues with the browser killing the parent early.