Set QSS-parsed gradients' coordinate system to ObjectBoundingMode

Set QSS-parsed gradients' coordinate system to ObjectBoundingMode

In this mode, gradient coordinates are relative to the bounding rect of the object
to be drawn. Thus, gradients can be specified between (0,0) and (1,1) in stylesheets
and will do The Right Thing™ regardless of the object's size.
This is also what Qt uses for its widgets.