docs: Q_CLASSINFO is used in a couple more places besides ActiveQt

Change-Id: I000f538dc11477224efbf2edc91873822eb9fef2
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Make HarfBuzz-NG the default shaper on Mac

For AAT shaping support, we need either the pre-QPA shaping trick
or the new HarfBuzz on Mac; prefer the latter.
Disable some test cases aimed to test the HB-old behavior; enable ones
that should guarantee shaping-unaware behavior.

[ChangeLog][OS X] Use CoreText text shaping engine for support of
complex scripts. If required, the shaping engine used in previous
versions can be preferred by configuring Qt with -no-harfbuzz.
Alternatively, the QT_HARFBUZZ environment variable could be set to "old".

Task-number: QTBUG-18980 (relates)
Task-number: QTBUG-38246
Change-Id: Iee6fe4f5bc047e77259182b8585385c5febd02b3
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>
Improve the Unicode script itemization implementation

Make it closer to the Unicode specs (UAX#24):
* Common now inherits the preceding character's script, if any;
* In a combining character sequence, if the base character is
  of Common script, the entire sequence is treated like if it were
  of the first non-Inherited, non-Common script in the sequence.
See http://www.unicode.org/reports/tr24/tr24-21.html for more details.

[ChangeLog][QtGui] Fixed regression in arabic text rendering.

Task-number: QTBUG-28813
Task-number: QTBUG-29930 (related)
Task-number: QTBUG-35836
Change-Id: Id85761965b08ca94c674d5f3613fe58b82b2ce9c
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>
Reviewed-by: Ahmed Saidi <justroftest@gmail.com>
Disable threaded rendering if Mesa is used

Mesa and xcb show some bad interaction which leads to frequent crashed
on multithreaded access. Also, the selective approach to blacklisting
only specific chipsets isn't feasible, given the resources available.

The client glx vendor string is used to identify mesa instead of the
server GL vendor and/or renderer string as that is much more reliable.

Task-number: QTBUG-38221
Change-Id: I2d8c037aa4fd9c38eb9537452a5e7e62f72a081d
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>