Improve read marker

- Update from deprecated readMarkerEventId to lastFullyReadEventId for quotient 0.7.
- Removed duplicate code for marking read from the read marker delegate.
- Make sure that mark as read only trigger at the end of the timeline
- Allow the read marker to be highlighted when jumped to

src/qml/Page/RoomPage.qml

@@ -649,7 +649,7 @@ Kirigami.ScrollablePage {
     // Mark all messages as read if all unread messages are visible to the user
     function markReadIfVisible() {
         let readMarkerRow = eventToIndex(currentRoom.readMarkerEventId)
-        if (readMarkerRow > 0 && readMarkerRow < firstVisibleIndex()) {
+        if (readMarkerRow > 0 && readMarkerRow < firstVisibleIndex() && messageListView.atYEnd) {
             currentRoom.markAllMessagesAsRead()
         }
     }