Expose location of .desktop file in environment

[TheOneRing]

Qt expects an application to specify the actual desktop file name https://doc.qt.io/qt-6/qguiapplication.html#desktopFileName-prop.
With AppImages, however, this is usually not know, or predictable, for the application.

The desktop file name is also required in the context of Freedesktop notifications https://specifications.freedesktop.org/notification/latest/hints.html, where it is used to manage the system integration of the notification settings.

CC: opencloud-eu/desktop#982

[TheAssassin]

Sounds like we'd have to either communicate the fixed name or reconsider renaming the desktop file altogether. The renaming however makes sure that multiple versions of the same AppImage can be used concurrently.