Add optional spellchecking via libspelling

[pvitt72]

Adds inline spellchecking backed by libspelling, with the dependency kept optional.

How it works

• Each document attaches a Spelling.TextBufferAdapter to its GtkSource.Buffer, which does the inline highlighting on its own.
• The Spelling.Provider and Spelling.Checker are module level singletons shared by every document, so adding or ignoring a word applies everywhere. invalidate_all_documents() then re-checks the open buffers.
• No LaTeX or BibTeX syntax is filtered in Python. libspelling skips whatever carries the GtkSourceView no-spell-check context class, so what gets checked is decided entirely in data/resources/language-specs/. This PR extends bibtex.lang to mark entry types, field names, citation keys and identifier fields such as url/doi/isbn.

Optional dependency

Document.init_spellchecker() catches ImportError/ValueError and leaves document.spellchecker as None, so Setzer runs unchanged when libspelling or the hunspell dictionaries are missing. Every caller — actions, context menu, preferences page — checks for None.

The README gets the extra packages needed to enable it (gir1.2-spelling-1, libspelling-1-1 and at least one hunspell dictionary).

User interface

• Preferences → Editor gains a Spellchecking section with an on/off toggle and a language chooser, backed by two new preferences in Settings.set_defaults(): inline_spellchecking and spellchecking_language_code (None meaning the system default).
• The right-click menu offers the suggestions plus Add to Dictionary and Ignore. The target word is taken from the pointer position rather than from the cursor, because right-clicking does not move the cursor in Setzer.

Note on po/POTFILES

No change was needed: it already lists setzer/document/spellchecker/*. Unrelated to this PR, that file is otherwise quite stale — running po/generate-potfiles.sh on current master changes about 60 lines. I left it alone to keep this PR focused, but you may want to regenerate it separately.