utils/serviceutils: Improve Exec key parsing
Take into account Exec field codes.
The change makes possible to have both X-KDE-Wayland-Interfaces field (for example, kde_lockscreen_overlay_v1) (and additional arguments for Exec field (for example, %U).
See also:
https://specifications.freedesktop.org/desktop-entry-spec/latest/ar01s07.html
Contributes to:
Edited by Alexey Andreyev