D-Bus activation systemd service

Correctly scopes dolphin in systemd service `app-org.kde.dolphin.service` when dbus-activated (while still working if systemd is not present), as per https://systemd.io/DESKTOP_ENVIRONMENTS/

uses new ECM modules:
- frameworks/extra-cmake-modules!5 to simplify the systemd service templating + install
- frameworks/extra-cmake-modules!17 to generate the dbus service file automatically

needs frameworks/extra-cmake-modules!25