Use semantically correct icon for Dolphin addon menu items

Using `kgpg` as the icons is not semantically correct and results in the
icons being illegible whith dark color schemes because `kgpg` is an app
icon and app icons don't change their colors, but it has the appearance
of a monochrome color-changing icon.

BUG: 420457
FIXED-IN: 20.08.0