Sync ("Wayland") appending to match SDDM

Summary:
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
match exactly.

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.

BUG: 417499

Test Plan: Opened KCM

Reviewers: Plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27358