Add "delete retained topic" button

app/src/actions/Sidebar.ts

@@ -0,0 +1,19 @@
+import { Dispatch, Action } from 'redux'
+import { AppState } from '../reducers'
+import { makePublishEvent, rendererEvents } from '../../../events'
+
+export const clearRetainedTopic = () => (dispatch: Dispatch<Action>, getState: () => AppState)  => {
+  const { selectedTopic, connectionId } = getState().tooBigReducer
+  if (!selectedTopic || !connectionId) {
+    return
+  }
+
+  const publishEvent = makePublishEvent(connectionId)
+  const mqttMessage = {
+    topic: selectedTopic.path(),
+    payload: null,
+    retain: true,
+    qos: 0 as 0,
+  }
+  rendererEvents.emit(publishEvent, mqttMessage)
+}