Workaround a bug in Core Text to select Light fonts

Workaround a bug in Core Text to select Light fonts

Currently in Core Text there is not proper way to select fonts with
Light weight, for example:
QFont font("Helvetica"); font.setWeight(QFont::Light);
will give you Helvetica-Light, as with:
QFont font("Helvetica"); font.setWeight(QFont::Normal);
because of a bug in Core Text, applying 0 symbolic traits with
CTFontCreateCopyWithSymbolicTraits will always return the Light
variant of that font family. Thus, we should only do this unless
symbolicTraits is not 0 or font.weight is not Normal (Light is not
a symbolic trait, but CT doesn't support selecting Light weight
numerically).

Reviewed-by: Eskil
(cherry picked from commit 4d5b8f66d82e9087d9d58a4e76e6b46ce7bb53cc)

Change-Id: I37a970aba5019a13b9f3bc43b7fb594b74a1aa37
Reviewed-on: http://codereview.qt.nokia.com/124
Reviewed-by: Jiang Jiang <jiang.jiang@nokia.com>