Make Qt examples self-contained

Qt examples should not reference individual source or asset files outside of their individual example directory. If an example uses a 'shared' directory, it either should duplicate the relevant sources and assets in the respective example directories, or create a static 'QtXYModuleExamplesPrivate' library that is built as part of Qt, and can be referenced by these examples - something we already discussed & implemented before e.g. for sharing icons.

https://www.mail-archive.com/development@qt-project.org/msg43753.html