Update the QSignalBlocker docs to show actual behavior

QSignalBlocker resets to the previous state, which might have already
been a blocked signal state.

Task-number: QTBUG-39352
Change-Id: I918cc6ea346755b940e027cee5d6704824fbba32
Reviewed-by: David Faure <david.faure@kdab.com>
Fix QSysInfo::macVersion for OS X minor versions greater than 9.

gestaltSystemVersion's encoding only has room for a single version
digit. Thus, OS X 10.10 would previously have been detected as OS X 10.9
(Apple's comments in the header even warn against this).

Change-Id: I41c35b1507d39e2958a9aaffaa8c48ac380f61d9
Reviewed-by: Samuel Gaist <samuel.gaist@edeltech.ch>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
QDialogButtonBox does not need TabFocus

It will now share the same behavior as QGroupBox, which has NoFocus
if its not checkable.

This doesn't lure AT clients to stop and read its content (this is the
case for android accessibility) (designer seems to generate a tooltip
for each QDialogButtonBox, which will be read as the name).

Change-Id: I6cfacdd9c01299521222c773634df1e36971d982
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>