Tidy up Info.plist.
Use feature availability checks instead of version checks.
Use Q_OS_MACX to refer to OS X because in Qt 5, Q_OS_MAC includes iOS.

Q_OS_OSX is available as of Qt 5.2 (therefore obviously not in 4.x).
Enable setUnifiedTitleAndToolBarOnMac for all Qt versions.

We changed our minds, and this will return in (possibly) 5.2.1. For the
versions in which it is not available, it is simply a no-op and
therefore safe to use in any version.

When QtMacExtras has a more native solution available (which will not
be backported to Qt 4) this can go in the #else of an ifdef for
QT_MACEXTRAS_LIB.
Refer to the OS X operating system by its current official name.
Whitespace fixes.