Originally on macOS it would override any tab text color with white
for the active tab as it would set it directly on the option palette
inside the style code. This would cause it to ignore any changes done
by the user in a changed option, via setTabTextColor or in a proxy
style. Therefore the setting of the color should be done when the style
option is initialized and only if the tab text color has not been
set by the user.
This has the added effect of making it easier to change the color for
tabs via a stylesheet since it will not be overridden in the style but
setup correctly in the option instead.
Change-Id: Ic338e96470112cba71d422bce79e664df0cb188a
Reviewed-by: Jake Petroules <jake.petroules@qt.io>
2c0033983b