Add support for dimming the colors when losing focus

Summary:
pretty vital (at least for me) if one doesn't have compositing.

we can't use the normal QPalette::Inactive because of color schemes,
but I made the new scrollbar color code follow the same dimming colors.

Test Plan:
tested with and without compositing, with and without scrollbar,
and dark and light color schemes.

Reviewers: hindenburg, Konsole

Reviewed By: hindenburg, Konsole

Subscribers: anthonyfieroni, konsole-devel, Konsole

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D16198