VS: Fixed CSharp VS project creation that contain .Designer.cs files.

This fixes a problem in CMake 3.8.0-rc1 for VS C# project creation involving auto-generated designer forms. The original implementation makes the assumption that the .Designer.cs file can only be paired with one other file of the same name but extension ending with .cs, .resx, .settings, or .xaml. This assumption is not correct. Designer forms can occur as file triplets with .Designer.cs, .cs, and .resx. The submitted patch fixes the problem for a VS C# application that was ported to the CMake build system.