Sync ("Wayland") appending to match SDDM
For historical reasons some .desktop files in wayland-sessions suffix
their .desktop files, some expect the display manager to do it.
SDDM compares the suffix and only adds if needed. Our config UI should
The obvious comment is this only works in English, but for some reason
both SDDM and this code only read the Name entry in English anyway.
Test Plan: Opened KCM
Reviewers: Plasma, ngraham
Reviewed By: ngraham
Subscribers: ngraham, plasma-devel
Differential Revision: https://phabricator.kde.org/D27358