LabelBackground: Implement better, Breeze-like styling

Breeze style uses a combination of Window/textColor and highlight color with varying alpha values to draw its QProgressBar with a nice border and a fitting background color.

Had to use hard-coded opacity component, because in QtQuick Qt.alpha function only overrides base color's alpha, but Breeze uses multiplication to obtain progressively smaller fractions. In the end of the day, it doesn't terribly matter, as long as it looks fine.