Fix & improve "Setting Up" section to match code and fix buildsystem fails
Fix & improve "Setting Up" section to match code and fix buildsystem fails

* create QPushButton on heap in first code example
  (as usually done, so will not raise eye-browse of Qt coders,
  also matches pattern later done with MainWindow
* use consistent code in CMakeLists.txt across examples snippets and file
* add main_window.cpp/hpp only to CMakeLists.txt once created
  and show how to add to CMakeLists.txt

Michael-F-Bryan/rust-ffi-guide,Michael-F-Bryan/rust-ffi-guide,Michael-F-Bryan/rust-ffi-guide