Cherrypick Other user read markers 23.04

Add the ability to see other user's read markers. 

![image](/uploads/a4bdc35a60ab10685c7d3ea4d9b13c87/image.png)

Implements network/neochat#133


(cherry picked from commit 23303c0483)

src/models/messageeventmodel.h

@@ -57,6 +57,9 @@ public:
         ShowAuthorRole,
         ShowSectionRole,
 
+        ReadMarkersRole, /**< QVariantList of users at the event for read marker tracking. */
+        ReadMarkersStringRole, /**< QString with the display name and mxID of the users at the event. */
+        ShowReadMarkersRole, /**< bool with whether there are any other user read markers to be shown. */
         ReactionRole,
 
         IsEditedRole,