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