Add support to grant advancements to offline players.

[tlm9201]

Would be nice to have some feature in the API for this. Currently I have to listen to an additional PlayerJoinEvent.

For example, an advancement based on how many chunks are in the town you're currently in.

Currently:

class ChunkClaimAdv extends BaseAdvancement {
// ...
 ChunkClaimAdv(/*...*/) {
  super();

  registerEvent(PlayerJoinEvent.class, event -> {
   // towny claim logic, get players town chunks
  });

  registerEvent(TownChunkClaimEvent.class, event -> {
   // additional towny claim logic, loop thru all ONLINE players in the town.
  } 
 }
}

With offline player support:

class ChunkClaimAdv extends BaseAdvancement {
// ...
 ChunkClaimAdv(/*...*/) {
  super();

  registerEvent(TownChunkClaimEvent.class, event -> {
   // towny claim logic, loop thru all online AND offline players in the town.
   // offline progression is applied by the API on player join
  } 
 }
}

[EscanorTargaryen]

Offline players are already supported, you can find more in the wiki.