Show special text when room has no events yet

e.g. if invited to a room but there's no history provided by
another account.

Also add a signal for the ListModel.count property.

harmonyqml/components/chat/MessageList.qml

@@ -1,6 +1,7 @@
 import QtQuick 2.7
 import QtQuick.Controls 2.2
 import QtQuick.Layouts 1.4
+import "../base" as Base
 
 Rectangle {
     property int space: 8
@@ -34,4 +35,17 @@ Rectangle {
             }
         }
     }
+
+    Base.HLabel {
+        visible: messageListView.model.count < 1
+        anchors.centerIn: parent
+        text: qsTr("Nothing to see here yet…")
+        padding: 10
+        topPadding: padding / 3
+        bottomPadding: topPadding
+        background: Rectangle {
+            color: "lightgray"
+            radius: 5
+        }
+    }
 }