Add method to get the current player-visibillity setting

src/main/java/de/bluecolored/bluemap/api/WebApp.java

@@ -51,6 +51,13 @@ public interface WebApp {
      */
     void setPlayerVisibility(UUID player, boolean visible);
 
+    /**
+     * Returns <code>true</code> if the given player is currently visible on the web-app.
+     * @see #setPlayerVisibility(UUID, boolean)
+     * @param player the UUID of the player
+     */
+    boolean getPlayerVisibility(UUID player);
+
     /**
      * Creates an image-file with the given {@link BufferedImage} somewhere in the web-root, so it can be used in the web-app (e.g. for {@link Marker}-icons).
      *