This has should be done explicitely, as the docs explain.

This causes issues for the KScreenLocker KCM port.

So that #include <kconfig_version.h> still works after changing its
location in the previous commit.

NO_CHANGELOG

This was built with:
-DQT_MAJOR_VERSION=6 \
-DEXCLUDE_DEPRECATED_BEFORE_AND_AT=5.90.0 \
-DKF_DISABLE_DEPRECATED_BEFORE_AND_AT=0x055a00

Move the include(KDEInstallDirs) call before the first find_package(Qt*,
the former is what auto-detects the Qt version, and defaults to 5. This is
needed to be able to build against Qt5 by default.

All unit tests still pass.

This way consumers which want to use the ConfigPropertyMap don't have to
pull in KDeclarative's entire dependency tree.

Also we can remove the automatic saving of the config, previously
this was opt-out - which makes is difficult to deprecate anything.

This way the API design is also more clear, since the object only takes care of exposing the
data to QML. The writing has to be done manually, which makes more sense anyways when we have
the notify opt-in.

As discussed on the KF6 weekly thread, an optional QML submodule is seen as the best
way to handle the QML dependency.

Task: https://phabricator.kde.org/T12131

Relates to https://phabricator.kde.org/T12126, after his change the KDeclarative stuff can be deprecated.
Because we don't register the property map in any QML plugin, there is
no conflict in duplicating and modifying it now.