I created a class library which is contained of WPF Windows and some user controls inherited from my c# classes that helps me to customize certain wpf controls.
Now
To transform a classical library project to a WPF library project (in order to add UserControls, Windows, ResourcesDictionaries, etc.) you can add the following XML in the .csproj file in the first PropertyGroup Node :
{60dc8134-eba5-43b8-bcc9-bb4bc16c2548};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}
Full example :
Debug
AnyCPU
{50E8AAEA-5CED-46BE-AC9A-B7EEF9F5D4C9}
Library
Properties
WpfApplication2
WpfApplication2
v4.6.1
512
{60dc8134-eba5-43b8-bcc9-bb4bc16c2548};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}
4
true