CMake project sources aren't properly grouped #16

Describe the bug
The source_group call is/was ill formed and didn't organize sources with source_group(TREE for IDE projects.

To Reproduce

1. Generate a new project based on the template on windows
2. Add a preset for the msbuild generator based on x64-windows-msvc
3. Invoke cmake with the preset
4. Open the generated solution with Visual Studio

Observe that all project source files are grouped into Header Files and Source Files directories.

Expected behavior
The project source files should be grouped in a tree similar to the filesystem layout.

Environment

• Project Version: v0.2.5