Slightly reduce the tinting for selected icons and previews

The intention of the tinting was to make it more obvious in icons view
which icons are selected. However, some icons and previews look quite
ugly with the current tinting value of 1.0 (i.e., the value passed to
KIconEffect::colorize). A slight reduction of this value to 0.8 makes
this a little less ugly. However, the real fix is to remove the tinting
altogether and find something better to indicate which items are
selected.