Add  Xwt backend

Anyone interested in [Xwt](https://github.com/mono/xwt) support? For a project I needed something more powerful than NPlot and tried OxyPlot. Since I have some platform independent projects, we use Xwt as a cross-platform solution for our Guis (it supports Gtk, Wpf and Mac for now). Xwt is used by XamarinStudio for example and is part of [MonoDevelop](https://github.com/mono/monodevelop/tree/master/main/external).

This PR adds OxyPlot.Xwt with some examples (GTK, WPF). Since Xwt can not be found on NUget (maybe sometime), I've added a git submodule (External/Xwt).

I hope that this PR fits the projects guidelines. Let me know if I should change or add something.