Please port to Qt6

[Nowa-Ammerlaan]

Downstream bug: https://bugs.gentoo.org/926671

We'd like to minimize the time during which we have to maintain both the Qt5 and Qt6 versions of QtWebEngine.

I gave it a shot myself, but got stuck here:

automatictheme.cpp: In constructor ‘AutomaticTheme::AutomaticTheme(QWidget*)’:
automatictheme.cpp:50:53: error: ‘updateTimeout’ is not a member of ‘QGeoPositionInfoSource’
50 |     connect(m_gPosInfoSrc, &QGeoPositionInfoSource::updateTimeout, this, [=]() {
|                                                     ^~~~~~~~~~~~~
automatictheme.cpp: In member function ‘void AutomaticTheme::on_refresh_clicked()’:
automatictheme.cpp:80:53: error: ‘class QDateTime’ has no member named ‘toTime_t’
80 |         sun.sunrise(QDateTime::currentDateTimeUtc().toTime_t()));
|                                                     ^~~~~~~~
automatictheme.cpp:82:52: error: ‘class QDateTime’ has no member named ‘toTime_t’
82 |         sun.sunset(QDateTime::currentDateTimeUtc().toTime_t()));
|                                                    ^~~~~~~~
make: *** [Makefile:1745: automatictheme.o] Error 1

My patch so far (just makes some adjustments for stuff that has been moved around in QtWebEngine): whatsie-4.14.2-qt6.txt