React to externally left and joined rooms, and add "leave room" button in room menu (#75)

* Initial "join room" feature.
* React correctly to remotely joined rooms.
* Leaving rooms implemented both locally using the room menu
   in nheko, and reacting properly when leaving a room remotely 
   from another client.

include/LeaveRoomDialog.h

@@ -0,0 +1,19 @@
+#pragma once
+
+#include <QFrame>
+
+#include "FlatButton.h"
+
+class LeaveRoomDialog : public QFrame
+{
+        Q_OBJECT
+public:
+        explicit LeaveRoomDialog(QWidget *parent = nullptr);
+
+signals:
+        void closing(bool isLeaving);
+
+private:
+        FlatButton *confirmBtn_;
+        FlatButton *cancelBtn_;
+};