Doc: Update, and reduce duplication of, QThread-related info

Added/Changed:
- Move content from the Thread Basics overview to the QThread class ref
- Rephrase bits for clarity
- Use more links

Removed:
- (threads-basics.qdoc) Warning against moveToThread(this): This usage
  came about when people tried to add slots to a QThread subclass. This
  patch adds a warning against the root cause.
- (threads-basics.qdoc) Note on sleep() et al.: They were made public in
  Qt 5.0.
- (threads-basics.qdoc) The strategy for managing member variables:
  Sounds error-prone. Pushing results through signals is safer.
- (qthread.cpp) The note about GUI classes: Irrelevant to QThread,
  and it's already mentioned elsewhere.

Change-Id: I6bc53cc22b929523f9976d2b920f94c02bd7273e
Reviewed-by: Geir Vattekar <geir.vattekar@digia.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>