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_7_R3/src/main/java/com/lishid/openinv/internal/v1_7_R3/PlayerDataManager.java

@@ -16,6 +16,8 @@
 
 package com.lishid.openinv.internal.v1_7_R3;
 
+import java.util.Arrays;
+import java.util.Collection;
 import java.util.UUID;
 
 import com.lishid.openinv.internal.IPlayerDataManager;
@@ -78,4 +80,9 @@ public class PlayerDataManager implements IPlayerDataManager {
         }
     }
 
+    @Override
+    public Collection<? extends Player> getOnlinePlayers() {
+        return Arrays.asList(Bukkit.getOnlinePlayers());
+    }
+
 }