Create a list model for readmarkers

Create a list model for read markers. The primary reason is to stop `RoomMembers` being accessed after their state event is deleted. With this the read marker doesn't pass and `RoomMember` objects to qml.

src/timeline/MessageDelegate.qml

@@ -82,16 +82,6 @@ TimelineDelegate {
      */
     required property var readMarkers
 
-    /**
-     * @brief String with the display name and matrix ID of the other user read markers.
-     */
-    required property string readMarkersString
-
-    /**
-     * @brief The number of other users at the event after the first 5.
-     */
-    required property var excessReadMarkers
-
     /**
      * @brief Whether the other user read marker component should be shown.
      */
@@ -342,8 +332,6 @@ TimelineDelegate {
             Layout.rightMargin: Kirigami.Units.largeSpacing
             visible: root.showReadMarkers
             model: root.readMarkers
-            toolTipText: root.readMarkersString
-            excessAvatars: root.excessReadMarkers
         }
 
         DelegateSizeHelper {