Add initial CMake support

[gruenich]

Not yet ready, but it compiles so far.

Increased or introduced required minimum versions:

• CMake 3.19
• Qt 6.2 or 5.15
• Compiler has to support C++17

Missing:

• Finding and using HAL
• Installation
• Add deprecation warning to qmake
• refactor: add support for Qt6 #36 is merged and this rebased
• Decide on support for Qt 5 and adding it

Fixes #37

[letsfindaway]

If you want to support both Qt 5 and Qt 6, here is an example how I did this in OpenBoard. This code automatically detects whether Qt 5 or Qt 6 is available. If both are installed, a variable can be used to decide. In OBS, you can just change the BuildRequires and CMake takes whatever it gets. Just a proposal, there are definitively also other ways.

I would like to have support for both. Even if we might only use Qt 6 in openSUSE in the future, other distros might want to continue using Qt 5.

Off-topic: How many changes are require to justify an increase in the version number?

[gruenich]

Sure, I will cook something up to get both Qt 5 and 6 supported.

Your changes + the new CMake build system would be enough. Every other open merge request should be included, too. Having a broken previous release and have it fixed is enough to justify a new release. Otherwise packagers need to fiddle with patches.

[gruenich]

I think this need some review and testing. Overall, it should do as advertised.