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>
tests: Replace Q_OS_MACX -> Q_OS_OSX

Use the correct identifier for the OS X operating system.

Change-Id: I7158a6b77e5e7418bc6b0a565f003500820a346d
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
changelog: add note about TLS session tickets

Change-Id: I154a56ff85adb8d43ddb730d91d2c59f129f7a26
Reviewed-by: Richard J. Moore <rich@kde.org>
iOS: default_post: let xcodebuild_distclean depend on xcodebuild_clean

Otherwise, make would upon distclean first remove the xcode
project, then try to do xcodebuild distclean. xcodebuild would
then complain about a missing project.

Change-Id: I0a9a6af6d86d1a95e37f4bbafa38c63d892bf1cc
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Fix Q_ASSERT(!channels[0].isSocketBusy());

Since commit f30641a7 is has been possible to issue more than one host
lookup request per HttpNetworkConnection. If the result was both an
IPv4 and IPv6 address, and we get a second similar DNS reply, we
end up triggering the assert in startNetworkLayerStateLookup().

This patch splits the InProgress state to HostLookupPending and the state
of trying both IPv4 and IPv6. This makes it possible to ignore any new DNS
replies received after the first succesfull one.

Change-Id: I0b8d6b1582fdaed69dde5926019b60bb0cbd580d
Reviewed-by: Peter Hartmann <phartmann@blackberry.com>