[snap] Spellchecking appears to be not working

Possibly related console output:

```
(gedit:16993): GLib-GIO-WARNING **: 22:40:02.951: Error creating IO channel for /proc/self/mountinfo: Permission denied (g-file-error-quark, 2)

(gedit:16993): gspell-WARNING **: 22:40:03.068: iso_639.xml: Failed to open file ?/usr/share/xml/iso-codes/iso_639.xml?: open() failed: No such file or directory

(gedit:16993): gspell-WARNING **: 22:40:03.068: iso_3166.xml: Failed to open file ?/usr/share/xml/iso-codes/iso_3166.xml?: open() failed: No such file or directory
```

The check spelling window spills "Error: Spell checker error: no language set. It’s maybe because no dictionaries are installed."

In the poedit snap that uses GtkSpell and Enchant to implement the spellchecking we patched the iso-codes prefix and packs all the spellchecking dictionaries to solve the issue:
https://github.com/vslavik/poedit/blob/master/snap/snapcraft.yaml

Also, I've packaged a Hunspell content snap that provides the dictionaries:
https://forum.snapcraft.io/t/the-hunspell-dictionaries-content-snap/7160