V2: I will try to make a branch of Qt6/C++17

The current C++11 is a little to restrictive, and for the Qt5/Qt6 side, some migration is necessary.

If you have any suggestion. Please let me know.

I'm communicating for a while now, that the latest 2.2.0 release will mark the end of V2.
My plans were to triage the remaining pull requests and issues for a final 2.2.1 release and then switch over to V3.

There are a couple of points we can't or shouldn't solve with the V2 code base, because the V2 code base has the QCAD license dependency, which is a big obstacle in many regards.
- the often mentioned performance issue with big DXF files
- more advanced support for DWG format
- TTF font support
- true scripting support (LISP/Python/LUA)
- and possibly more

So from my point of view, I see no benefits in modernizing the V2 code base.
I think with the current 2.2.0 release we have the most stable and feature rich LibreCAD I know, and this gives us the ability to focus on V3, push it to a usable 3.0 release state and make it a honorable V2 successor in the future.

I just recently declined an offer for Qt6 migrations from an asking, new contributor, with this argumentation.
Sadly, he didn't join us, neither for V2 nor V3.

But we have to point that out, to avoid big disappointments when anybody starts to put some amount of efforts into V2 code base, which will probably become obsolete sooner or later.

Armin