* 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.
