entryArea: Share a common spell checker

We currently use GSpell's convenience API to add spell checking
support to entries, which turns out to be quite wasteful when
used with lots of separate entries:
Every entry gets a separate GSpellChecker object with its own copy
of the dictionary.
Address this by setting up all entries manually with a shared spell
checker object.

https://bugzilla.gnome.org/show_bug.cgi?id=780762