Doc: Discuss the concept of thread affinity in more detail

Doc: Discuss the concept of thread affinity in more detail

- Create a section dedicated to this concept, which is fundamental to
  signal/event handling
- Move relevant content from the very broad "Thread Basics" page to the
  QObject class ref
- Flesh out existing content, including distinguishing signals from
  events
- Address the common misconception that "member" = "child"; this has
  been encountered several times in the Qt Project forums

This is a cherry-pick from 3b45dfe6e6ff6c0626b in qtbase.git

Change-Id: If3b65f3da4e1ca414c7175c4e22bcdef38f45442
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>