Make all warnings fatal in CI builds

This MR:

• enables a feature that enforces all compiler/linker warnings reported as errors (-Werrors or /WX, depending on compiler)
  • also replaces set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}") calls with generic add_compile_options().
  • also adds -Werror=return-type to regular (i.e. non-Werror) gcc/clang builds. En equivalent C4716 on msvc is already an error by default.
    • The -Werror= list could be extended further for known warnings that we otherwise want to be firm failures.
• enables that feature in CI builds
• also enables CMake's own warnings to be reported as errors in CI builds
  • requires having non-REQUIRED find_package() calls be QUIETed to not throw the CMake Module/Config not found warnings
  • this, however, seems to be affecting cmake_dependent_option() calls, see: https://gitlab.kitware.com/cmake/cmake/-/issues/23959
• enables additional model tests in CI builds
• fixes compilation warnings with gcc, clang and msvc
• fixes CMake warnings, particularly regarding outdated/missing policy declarations
• adds missing KF5 dependencies to .kde-ci, in order to enable compilation of optional plugins
  • some plugins, which depend on autotools libs, lik aqbanking, are still not available on some platforms, though