Fix getOnlinePlayers call to support all versions

Moved IPlayerDataManager from api module to common module - it is not part of the API as there is no supported way to obtain an instance of it.

internal/v1_8_R3/src/main/java/com/lishid/openinv/internal/v1_8_R3/PlayerDataManager.java

@@ -16,6 +16,7 @@
 
 package com.lishid.openinv.internal.v1_8_R3;
 
+import java.util.Collection;
 import java.util.UUID;
 
 import com.lishid.openinv.internal.IPlayerDataManager;
@@ -79,4 +80,9 @@ public class PlayerDataManager implements IPlayerDataManager {
         }
     }
 
+    @Override
+    public Collection<? extends Player> getOnlinePlayers() {
+        return Bukkit.getOnlinePlayers();
+    }
+
 }