linux: Use app_id as filepath for desktop file (#11337)

This undoes the changes from #11333 and uses the path of the `.desktop`
file instead.

According ot the spec
(https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html)
the filename and path of the `desktop` file are used to determine the
desktop file ID.

This is enough to match the windows (which have the same WMClass/app-id)
with the desktop entry.

Release Notes:

- N/A