add labels on buttons (fix #311)

src/OpenerButton.vue

@@ -15,11 +15,16 @@ You should have received a copy of the GNU Affero General Public License
 along with this program. If not, see <http://www.gnu.org/licenses/>.
 -->
 <template>
-  <button class="side-menu-opener"></button>
+  <button class="side-menu-opener" :arial-label="label"></button>
 </template>
 
 <script>
 export default {
   name: 'OpenerButton',
+  data() {
+    return {
+      label: t('side_menu', 'Toggle menu'),
+    }
+  }
 }
 </script>