This commit moves the – previously disabled – `auto-dictionary` code to
the `spell-checking` layer and activate it. It is now enabled by
default, but can be disabled by setting `spell-checking-auto-dictionary`
to `nil`.
`auto-dictionary` comes with some interesting improvements:
- Tries to automatically detect what is the language of the buffer and
activate the corresponding dictionary.
- When a buffer is opened, all spelling mistakes are shown without
needing first to be under the cursor.
- When the dictionary is changed, the whole buffer is reanalyzed for
spelling mistakes (close#2088).
