Change the title of spoiler button by state

The title will give the precise representation of the current state. It
would be helpful for blind people.

app/javascript/mastodon/features/compose/containers/spoiler_button_container.js

@@ -4,12 +4,13 @@ import { changeComposeSpoilerness } from '../../../actions/compose';
 import { injectIntl, defineMessages } from 'react-intl';
 
 const messages = defineMessages({
-  title: { id: 'compose_form.spoiler', defaultMessage: 'Hide text behind warning' },
+  marked: { id: 'compose_form.spoiler.marked', defaultMessage: 'Text is hidden behind warning' },
+  unmarked: { id: 'compose_form.spoiler.unmarked', defaultMessage: 'Text is not hidden' },
 });
 
 const mapStateToProps = (state, { intl }) => ({
   label: 'CW',
-  title: intl.formatMessage(messages.title),
+  title: intl.formatMessage(state.getIn(['compose', 'spoiler']) ? messages.marked : messages.unmarked),
   active: state.getIn(['compose', 'spoiler']),
   ariaControls: 'cw-spoiler-input',
 });