Recycle chat pages if possible when switching room

The chat page is complex and slow to create, which creates a visible lag
when user switches room.

Instead of throwing the pre-switch one away and
making a new one from scratch, keep the same page and update its user &
room ID for a big improvement in responsiveness. The rest is automatic
thanks to QML property bindings.

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

@@ -574,6 +574,11 @@ Rectangle {
             id: contextMenu
             eventList: eventList
         }
+
+        Connections {
+            target: chat
+            function onPageIdentityChanged() { eventList.moreToLoad = true }
+        }
     }
 
     Timer {