Merge Ookii Dialogs WPF and WinForms repositories and share common code

[augustoproiete]

Ookii Dialogs WPF and WinForms have a lot of code in common that is duplicated across two repositories (this one, and ookii-dialogs-winforms) which is harder to maintain and keep in sync and ensure feature parity.

Lets merge these two repos into one, and have the common code shared. We'll continue to have separate assemblies and nuget packages for WPF and Windows Forms, for everything that is specific to each UI framework.