Summary:
This is a preparation step to unset QT_QPA_PLATFORM from the wayland
startup session script. Setting QT_QPA_PLATFORM breaks 3rd-party Qt
software which does not bundle QtWayland. Most prominent example is
the Qt installer itself (see
https://bugreports.qt.io/browse/QTBUG-60222).

On the other hand our Plasma workspace applications need to be forced to
Wayland on a Wayland system. So we have a conflict between we want to
set QT_QPA_PLATFORM and we don't want to set QT_QPA_PLATFORM.

This change adds new API to KWorkspace to address this problem. The new
method adjusts the QT_QPA_PLATFORM based on the XDG_SESSION_TYPE
enviornment variable. It is completely opt-in. Meaning applications need
to explicitly add the call prior to creating the QGuiApplication and if
the user specifies either QT_QPA_PLATFORM env variable or any of the
-platform command line argument variants, the platform detection is
skipped.

The change also adjusts all plasma-workspace applications which should
use Wayland on Wayland to use the new API. The startup script on the
other hand still sets QT_QPA_PLATFORM. We also have applications outside
of plasma-workspace which needs this detection. Examples are:
 * powerdevil
 * systemsettings
 * kinfocenter

Once this change is merged those applications can be adjusted by linking
against PW::KWorkspace and afterwards QT_QPA_PLATFORM can be unset from
startplasmacompositor.

Test Plan: See added autotest

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10816