Allow fetching user defined dictionaries

[Olivier Patry]

• Drop any words_[a-z]{2}_[\d]+.txt in app's data dir (esp. for Desktop)
• Add a button to open a file picker in DictionaryPicker (would need CompositionLocalProvider for Desktop & Android)
• Define a builtIn attribute in Dictionary
• Add special indicator (user icon vs system icon?) in dictionary list
• Handle invalid data