Don't destroy formatting when editing previous messages

Adds a few new methods to grab the markdown/slightly rich text from the
message, and will intelligently re-insert user mentions as needed.

src/eventhandler.cpp

@@ -280,6 +280,22 @@ QString EventHandler::getPlainBody(bool stripNewlines) const
     return getBody(m_event, Qt::PlainText, stripNewlines);
 }
 
+QString EventHandler::getMarkdownBody() const
+{
+    if (m_event == nullptr) {
+        qCWarning(EventHandling) << "getMarkdownBody called with m_event set to nullptr.";
+        return {};
+    }
+
+    if (!m_event->is<RoomMessageEvent>()) {
+        qCWarning(EventHandling) << "getMarkdownBody called when m_event isn't a RoomMessageEvent.";
+        return {};
+    }
+
+    const auto roomMessageEvent = eventCast<const RoomMessageEvent>(m_event);
+    return roomMessageEvent->plainBody();
+}
+
 QString EventHandler::getBody(const Quotient::RoomEvent *event, Qt::TextFormat format, bool stripNewlines) const
 {
     if (event->isRedacted()) {