Add StartupWMClass to .desktop file

The org.kde.konsole.desktop file is missing a StartupWMClass setting.

BUG: 372441
FIXED-IN: 18.12

Differential Revision: https://phabricator.kde.org/D16546

(cherry picked from commit 1506ecdfd0f91d4b8b39c01bad1fe301193d6e82)