Qt6 Migration

The source files are modified to adapt to Qt6.
The CMake files are adapted to use Qt6.
`.gitlab-ci.yml` is adapted to find and use Qt6.
The Windows installer is tested successfully.

See merge request !18

`QMouseEvent.x()` is deprecated since Qt6; `QMouseEvent.position().x()` must be used instead.

Qt5.15 documentation reports that the results of `QFontMetrics::width()` were
inconsistent and unreliable in many situations, so it is deprecated.

See <https://doc.qt.io/qt-5/qfontmetrics-obsolete.html#width>

The properties `setConfirmOverwrite` and `setReadOnly` are obsolete.

Recovering old documentation

See merge request !17