Port sidebar delegate to KColorScheme.

Certain KDE color schemes have Window Background and View Text
not contrasting, which causes invisible icon labels when using
QPalette::Text on QPalette::Base. An example is the default
color scheme in openSUSE 12.3.

Thanks to Will Stephenson for noticing.
CCMAIL:wstephenson@kde.org

It turns out that the Places sidebar in Dolphin suffers from
the same bug, so a similar change should be done there. Frank?
CCMAIL:frank78ac@googlemail.com