Improve handling of offline players list

[Christian Illy]

Currently offline players list is transferred completely every few seconds (even if they are not selected on the map). This can cause a lot of unnecessary data transfer.

Ideal solution: Transfer the whole list once when opening the map and afterwards only transfer incremental changes when a player goes offline (when player goes online he'll be in the next online players list and can thus be removed that way from the offline list on the browser).

Alternative solution: Only transfer the (whole) list when offline players are selected in the map and add a way to only query the current offline count to display that number on the checkbox label.

[Christian Illy]

Done in webmap v23