Add (more) support code for IDEs (VS, Xcode)

[friendlyanon]

The gif_engine example project has such support code:

• Add headers to targets so they show up in solution view (ref)
• Use source_group to make solution view not flatten the files (ref)
• Add startup target, so by default people can debug right away conveniently (ref)

The folders.cmake is already IDE specific support code. The example project uses the target_sources_grouped command to combine target_sources and source_group, so files don't have to be mentioned twice or added to a list, which makes use more declarative. This is a command would have to be used on the main code path as well, which could make reading the project CML more difficult.