start-pulseaudio-x11: fix KDE version check
Current check wasn't working because it tried to grep $DESKTOP_SESSION.desktop in work dir which doesn't contain any $DESKTOP_SESSION.desktop files.
It also relied on $DESKTOP_SESSION env variable which isn't guaranteed to exist if session is started without display manager like sddm.
New approach greps hardcoded path /usr/share/xsessions/plasma.desktop if it exists with assumption this is where .desktop file for KDE plasma is installed.
This also changes legacy backticks to modern $(...) as recommended by shellcheck: https://github.com/koalaman/shellcheck/wiki/SC2006
Fixes #751 (closed)