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
Cope with global color scheme changes in the page number entry.

CCMAIL:wstephenson@kde.org
Get rid of window background-colored bars between thumbnail containers.

This seems to be what the original code wanted to achieve, but failed
due to a convoluted approach of setting the color of the current back-
ground role to a default palette's Base color ... instead we just leave
the palette untouched and set the correct background role now. Visual
noise be gone, yay.

CCMAIL:wstephenson@kde.org
Move the page numbers a little higher.

Make sure there's no ambiguity that the page number refers to
the preceding thumbnail. This is a bigger deal because the fix
in b0f64d2 took away a visual divider.
Remove unused member, thanks to Albert for noticing the slip-up.
Txt backend

BUGS: 261930
REVIEW: 109009
FIXED-IN: 4.11.0
Fixlets to the txt backend