Allow setting the spell checking language per-tab.

Above the input bar context menu item to enable automatic
spell checking is now a submenu to select among the avail-
able dictionaries. The chosen language is remembered for
server, channel and query tabs across application restarts.

Along the way this commit also fixes numerous bugs in
remembering the encoding settings for these tabs across
restarts, as some of the underlying code is shared and
the bugs were discovered during testing.