Move from Qt5 to Qt6

I have spent a few weeks working on this update. I have moved the graphical interface framework from Qt5 to Qt6 today.

While I have done a lot of testing to iron out any bugs from the move, it is possible bugs are present. Please post any bugs you find on the github issues page.

There are a couple of other update and error fixes that I have addressed also. I will probably put out a new 3.0 release in a month's time, to allow any Qt6 bugs to surface first.