Use qtpaths isntead of kf5-config in startkde

Currently env and shutdown scripts in startkde are not run due to the way kf5-config is used. There is no more $KDEHOME, so proper paths aren't being found.
This patch moves things to qtpaths, putting things under ConfigLocation + "plasma-workspace" + "env" (or "shutdown").

As a plus, kf5-config is no longer used in startkde.

CCBUG: 333793
REVIEW: 119145