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:

plasma-mobile/plasma-dialer!105