Fix event context menu targeting wrong message

If user right clicked on the most recent event in the timeline
(index 0), a new message was posted in the room (the new index 0),
and the user right clicked on it, the EventContextMenu's `event`
property relying on the index would not detect any change and
thus would still target the previous event.

This affected options depending on this property like "Reply".

The problem was reproducible with any same index repetition, not just 0.

src/gui/Pages/Chat/Timeline/EventContextMenu.qml

@@ -34,7 +34,10 @@ HMenu {
     }
 
 
-    onClosed: hoveredLink = ""
+    onClosed: {
+        hoveredLink = ""
+        eventIndex  = -1
+    }
 
     HMenuItem {
         icon.name: "toggle-select-message"