Place your methods into a series of Modules one for entirely generic methods, one for desktop only, one for WE only, etc (you need to do this since a method receiving or returning a WE object will not compile in a desktop app and vice versa. Code inside a method can be inside #if target commands).
Make these modules external to your project. As you create a new app, drag them as aliases into your new project. Now your interface is environment specific but your code is not.