Support of Qt6 (in addition to Qt5)

[hpretl]

mcy currently only seems to support Qt5 for the GUI part. Please extend it so that also Qt6 is supported. If Qt5 and Qt6 support is not possible, please consider moving to Qt6.

[hpretl]

@mmicko Is this something you could support?

[mmicko]

@hpretl main challange is updating 3rdparty libs to support Qt6. To be honest did not have much spare time to spend on this. Understand issue at your end. Would do my best to support Qt6.