Fix Name conflict between plasma.desktop and plasmawayland.desktop

Summary:
SDDM until 0.18.1 appends " (Wayland)" to the name of any session file it
finds in wayland-sessions dir, and Plasma is relying on that behaviour to
distinguish between its X11 and Wayland sessions. This leads to duplicate
"Plasma" entries on any other DM not applying the same hack, e.g. lightdm,
and users are lost without downstream patching.

SDDM in 0.19 only appends " (Wayland)" in case the session name does
not already end with this, to avoid duplicating it.

CCBUG: https://bugs.kde.org/show_bug.cgi?id=368409
See also: https://github.com/sddm/sddm/pull/997

Reviewers: Plasma, davidedmundson, fvogt, ngraham

Reviewed By: Plasma, davidedmundson, fvogt, ngraham

Subscribers: ngraham, pino, rdieter, fvogt, davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22210