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

Change-Id: I5e7e6c848596e72c0bb623f8442c8389b81ad9ef
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Doc: Rewrite section on threaded event loops

- Focus on signals instead of events; programs rarely need to call
  QCoreApplication::postEvent() manually
- Mention QMetaMethod::invokeMethod()
- Reduce verbosity

Change-Id: I170b96bd0134c0bc102ef1a344d4f0b88e504f86
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Doc: Show both low- and high-level thread syncing on the same page

Move text from "Thread Basics" to "Synchronizing Threads" for a more
complete overview.

Change-Id: Ib87259ed551fa77278b57c5922df7f4b774a829a
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Doc: Remove convoluted details of uncommon QMutex use case

Change-Id: If80068f44fef7ab0692b9dad99aa7e1a30cb72a6
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Doc: Generalize problems and solutions of mutex use

These apply to other locks too, not just mutexes.

Move from "Thread Basics" to "Synchronizing Threads"

Change-Id: I6d7051cb225a8c836fb591a28b65d3de8fab4083
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Doc: Expand on thread synchronization details

- Introduce the concept of "mutual exclusion"
- Rewrite/add explanations on how synchronization happens and how to
  use these tools
- Remove similar content from the "Thread Basics" page
- Fix links to examples

Change-Id: Id008a8fc3f68bf242cae1704c5c8318149d908b4
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>