If $HOME isn't canonical (e.g. on FreeBSD it's /home/user while the
canonical path is /usr/home/user), replacing the canonical version of $HOME
with $HOME means that we'll read back a different value than we wrote
in.
It might seem "equivalent" but it leads to surprises like
KRecentDocuments showing duplicates because /usr/home/user became
/home/user in the KConfig roundtrip (but not in the XBEL roundtrip).

This commit loses the replacement of /usr/home/user with $HOME on
FreeBSD, but I think an exact roundtrip is what we expect, rather than
stuff being modified under our feet. The alternative would be to
canonicalize everything in KRecentDocuments but users don't want to
see the /usr in front, I assume (so we would have to use a cache of
canonicalized path, for the removal of duplicates, awful performance
wise).

This is a followup to commit 3c861a6c3860d516

pyqt broke sip4 compatibility in 5.15.6, and there is no more maintainer
of the KF Python bindings to fix things.

Future support might need different code, so no advantage in keeping the
old code around.

This is basically the C++ counter-part to
https://invent.kde.org/frameworks/kconfig/-/merge_requests/94
and allows to easily retrofit window size persistence on existing windows/
dialogs, replacing e.g. code like
https://invent.kde.org/pim/pimcommon/-/blob/master/src/pimcommon/widgets/kpimprintpreviewdialog.cpp.

This is a bit more complicated than one might expect, as KWindowConfig
works with QWindows, but that's something freshly created QWidget windows/
dialogs don't have yet. Additionally, we are in a library here that doesn't
depend on Qt::Widgets. To overcome this we move the widget-dependent code
(basically just a call to QWidget::windowHandle()) to inline template code
(and thus into the consumer), use std::function's type erasure to pass it
into the library code, and an event filter on the widget to wait for the
QWindow to become available.

Since we are not building the library but the source files we
don't want the macro to expand to __declspec(dllimport).

It will trigger side effects like triggering the dbus signal
which depending on the timing may be delivered only in the next
test case causing an unexpected change signal emission.

Currently an application needs to be restarted before it can see
any changes made to the standard shortcut configuration. To notify
about changes a new class is introduced that looks for those
changes using KConfigWatcher and also updates the global map.
CCBUG:426656

In Qt6 the Q_GLOBAL_STATIC will already report to be null while it is in
the process of being deleted, we therefore cannot access it anymore from
destruction code path as we did before.

This problem is hit for example by the Breeze style, making all 6 based
widgets applications crash on exit without this.