Qt6: port to Qt6 - C++ part

[moodyhunter]

This PR tries to port QHot to Qt6, with:

Limitations:

• QtQuick.Dialog :: FileDialog has been moved to Qt.labs.platform (Available since Qt 5.8)
  • fileUrl property is now named file
  • (move it now?)
• QtGraphicalEffects is removed, may need Qt5Compat to import it back
  • (is this import really in use?)

Changes:

Added a CMake option QHOT_USE_QT6 which defaults to OFF
Used ${QT_VERSION_MAJOR} in CMake target_link_libraries and get_target_property which will be set to 5 or 6 accordingly.
Removed command line option scaling and no-scaling since HiDPI scales are always enabled in Qt6.

• Codes related to the limitations above are not changed (may be we cannot support both 5 and 6 ATST)

[patrickelectric]

Hi, just a simple question, this Qt6! is a especial tag for the compiler/interpreter or just a comment ?
Other than that the patch looks fine, but I'll probably have the master branch for Qt6 and create a separated branch for Qt5.

[moodyhunter]

Oh, I've seen this type of comment in some other projects, and just followed their rule.
It doesn't have a special meaning but makes the future search easier (like the keywords "// TODO" or "// WARNING")

[patrickelectric]

Hi @moodyhunter, any update on this ? I'm planning to do it this year otherwise

[moodyhunter]

I haven't been working on these Qt things recently, so there won't be future updates.

Do feel free to merge this PR if you think it's fine, otherwise closing it would be fine :)

[patrickelectric]

Thanks @moodyhunter, I'm doing the update to Qt6, the code works, has some warnings, but I'm closing up now the build system. Thanks for the PR, I'll check pick some commits if necessary.