store composer state when typing in new composer

this doesn't use the MessageComposerStore on purpose so that
both the new and old composer don't overwrite each others state,
as the format is different.

src/components/views/rooms/BasicMessageComposer.js

@@ -52,6 +52,7 @@ function selectionEquals(a: Selection, b: Selection): boolean {
 
 export default class BasicMessageEditor extends React.Component {
     static propTypes = {
+        onChange: PropTypes.func,
         model: PropTypes.instanceOf(EditorModel).isRequired,
         room: PropTypes.instanceOf(Room).isRequired,
         placeholder: PropTypes.string,
@@ -91,6 +92,10 @@ export default class BasicMessageEditor extends React.Component {
         this.setState({autoComplete: this.props.model.autoComplete});
         this.historyManager.tryPush(this.props.model, caret, inputType, diff);
         TypingStore.sharedInstance().setSelfTyping(this.props.room.roomId, !this.props.model.isEmpty);
+
+        if (this.props.onChange) {
+            this.props.onChange();
+        }
     }
 
     _onInput = (event) => {