[OS X] [Spellcheck] Dictionaries are not searched for in a user application bundle

Currently on OS X, spellcheck dictionaries are searched for in the following priority:
1) Use system-available dictionaries.
2) Search for dictionaries in the QtWebEngineCore framework bundle, specifically in Resources/qtwebengine_dictionaries.

But if a user wants to provide his own dictionaries, the only way he can do it is by copying them into the the Qt framework bundle, which does not seem ideal.

The other platforms have a fallback of looking into the application directory path. It would be useful to also search in the user's custom OS X application bundle (under Resources/qtwebengine_dictionaries).