Be explicit about X11 or Wayland for the dev session names

Summary:
I just switched to openSUSE Tumbleweed and noticed that the session names in SDDM's
chooser don't include "X11" or "Wayland" like they do in other distros I've used before. This
makes it impossible to tell which one you're going to boot into.

This patch adds those identifiers into the translated string so it's always clear.

Ideally https://github.com/sddm/sddm/pull/997 would get done first, but even it it doesn't,
it's no big deal.

Reviewers: fvogt, davidedmundson, apol, #plasma

Reviewed By: davidedmundson, #plasma

Subscribers: fvogt, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24666