Split xFunc UI to separate repository

[sys27]

xFunc UI is a windows-based application (WPF) that requires Windows OS to be able to build it. Also, the UI is kind of deprecated (we try to maintain it, but we don't add/update any of its features). So, each time when we add a new feature to xFunc.Maths we need to keep in mind that we need to update xFunc UI, which is not so critical to do (we can postpone the release of UI).

We need to try to split xFunc UI to separate the repository and remove all direct references to the xFunc.Maths library and replace them with NuGet packages. It will allow us to decouple UI from the main library and update them independently.