Remind the user if they're trying to reply to someone who has left

This happens semi-frequently to me and others - we reply to a user who
has left the room. Sometimes this is useful (for example, bringing up a
previous topic) but in most cases this is accidental, and almost
guaranteed to happen if you turned off join/leave events.

So I added a reminder when replying - which manifests as a small label
in the chatbar - that the user has left and can't be notified of your
reply.

src/libneochat/chatbarcache.h

@@ -99,6 +99,13 @@ class ChatBarCache : public QObject
      */
     Q_PROPERTY(Quotient::RoomMember relationAuthor READ relationAuthor NOTIFY relationIdChanged)
 
+    /**
+     * @brief If the author for the message being replied to is still present in the room.
+     *
+     * @sa Quotient::RoomMember
+     */
+    Q_PROPERTY(bool relationAuthorIsPresent READ relationAuthorIsPresent NOTIFY relationAuthorIsPresentChanged)
+
     /**
      * @brief The content of the related message.
      *
@@ -153,6 +160,7 @@ public:
     void setEditId(const QString &editId);
 
     Quotient::RoomMember relationAuthor() const;
+    bool relationAuthorIsPresent() const;
 
     QString relationMessage() const;
 
@@ -196,6 +204,7 @@ Q_SIGNALS:
     void threadIdChanged(const QString &oldThreadId, const QString &newThreadId);
     void attachmentPathChanged();
     void mentionAdded(const QString &mention);
+    void relationAuthorIsPresentChanged();
 
 private:
     QString m_text = QString();