Base version is 6.2, to support current Ubuntu LTS (22.04) release for
CI.

Besides the fairly obvious CMake stuff, this required:

- QRegExp removed -> now QRegularExpression with some different logic
- QRegularExpressionMatch::capturedRef removed -> replaced with
capturedView()
- QList<T>::toSet() removed -> replaced with QSet<T>(list.begin(),
list.end())
- QFileInfo(QString) is now explicit
- QDirModel removed; replaced with QFileSystemModel
- QVariant::type() deprecated, replaced with typeId() comparing against
QMetaType::Type
- QString::fromUtf16<ushort*>() deprecated, replaced with its char16_t*
overload

Other minor clean-ups are also included.

And in packaging:
- Depends on some new system dlls; exclude those from packages
- Also search mingw dlls from the CMAKE_FIND_ROOT_PATH.
  The Qt6 packages install dlls in an isolated subdirectory, so
  that's not enough to resolve e.g. libfreetype and some others.