* src/client/application/geary-config.vala (Configuration): Remove
  spell-check setting, we can just get it from the list of visible
  languages instead. Update the schema.

* src/client/components/client-web-view.vala (WebView::init_web_context):
  Pass in a config object, use that to init WebKit's spell checking on
  the WebContext now that is a global configuration, update it when the
  config changes, update call sites.

* src/client/composer/composer-widget.vala (ComposerWidget): Remove
  WK1-syle spell checking settings prefs.

* src/client/composer/spell-check-popover.vala (SpellCheckPopover): Pass
  a config object in so we don't have to use the global app singleton
  instance.

* src/client/dialogs/preferences-dialog.vala (PreferencesDialog):
  Modernise by converting into a widget template.

* test/client/components/client-web-view-test-case.vala (TestCase):
  Construct a config object as a fixture, use it to init the WebContext
  and make it avalaible to subclasses & update subclasses.

* ui/preferences-dialog.ui: Moved from preferences.dialog, remove spell
  check preference.