The setColor() function in QQuickText calls markDirty() which
did not specify that the whole paint node should be updated, so
color updates would have no effect until something else changed.

Change-Id: I93741b6c697f2dd6ad5a315250502a7a6e3ce1d7
Reviewed-by: Jiang Jiang <jiang.jiang@nokia.com>
Reviewed-by: Andrew den Exter <andrew.den-exter@nokia.com>