Fix wrong text color in Places Group Header. Use QPalette::Window for base color...

Fix wrong text color in Places Group Header. Use QPalette::Window for base color and QPalette::WindowText for text color. Also changed m_roleColor color mixing to 60% (from 70%) -> Better visible color difference when base color is darker than text color. Also changed styleOption().palette.brush(group, role).color() to styleOption().palette.color(group, role) in KStandardItemListWidget -> should be more efficient.

BUG: 303133
FIXED-IN: 4.9.1
(cherry picked from commit b3c3da985159a9627c079ad615cd77fc5f7bb72a)