Configure CMake to compile with warnings as errors

[mposs00]

Necessary to prevent usage of deprecated APIs and to enforce code quality

Depends on resolution of #798 #799 #800

[oldmud0]

Werror is not helpful because warnings are inconsistent across compilers. They can prevent someone else from building code, but they may be helpful for ci builds since we keep our toolchain versions fixed (or do we). On the other hand, which warnings have actually indicated a bug in the code, versus simply being nudged to use Qt6/C++20?

[mposs00]

q(s)rand() are both deprecated as of Qt5.10, QString::SplitBehavior is definitely deprecated in the Qt 5.15 LTS, idk about any sooner releases, I'm not sure when it was deprecated. I can test the build on clang, but the reported warnings were generated by GCC 11.3.0

[mposs00]

Defining the spec as C++11/17/whatever might be wise

[oldmud0]

I'm fine with setting the spec to c++17 and enabling -Werror on ci builds once we eliminate the compiler warnings we have.