message: Remove all text when cutting in the composer
  The previous function did in fact remove the elements, but left the model thinking there was a zero-length string. This approach deletes the text from the model and the DOM, resulting in the placeholder being shown when cutting all the text.
  Part of
diff: @@ -209,8 +209,9 @@ export default class BasicMessageEditor extends React.Component {
  const selectedParts = range.parts.map(p => p.serialize());
  event.clipboardData.setData("application/x-riot-composer", JSON.stringify(selectedParts));
  if (type === "cut") {
  - selection.deleteFromDocument();
  - range.replace([]);
  + // Remove the text from the composer
  + const {caret} = getCaretOffsetAndText(this._editorRef, selection);
  + this.props.model.update("", event.inputType, caret);
  }
  event.preventDefault();
  }
  