Change the "configure room" button icon to something more fitting

This button doesn't actually configure anything, you can do plenty of
actions like "mark as read" and such. Since the button isn't solely for
configuration, we should use an overflow menu icon instead.

src/qml/RoomDelegate.qml

@@ -145,7 +145,7 @@ Delegates.RoundedItemDelegate {
             text: i18n("Configure room")
             display: QQC2.Button.IconOnly
 
-            icon.name: "configure"
+            icon.name: "overflow-menu-symbolic"
             onClicked: createRoomListContextMenu()
         }
     }