Make handling of QtWebEngine dictionaries more flexible
This commit does three things:
- it adds a cmake option to explicitly turn off dictionary generation. This allows distributions which want to package the dictionaries separately to avoid building them without needing to use hacks such as passing -DCMAKE_DISABLE_FIND_PACKAGE_Hunspell=ON to cmake
- it fixes a bug causing the QTWEBENGINE_DICTIONARIES_PATH environment variable to be ignored by SpellCheckerManager, leading to dictionaries being searched in the wrong directory
- it allows Konqueror to fall back to QtWebEngine behavior if dictionary generation has been turned off and the QTWEBENGINE_DICTIONARIES_PATH environment variable is unset