Add tab player list header / footer API.

This adds a useable API for the player tab list header/footer for BungeeCord. (See issue #1201.)
Mojang doesn't seem to provide a way to clear the header/footer so instead an empty line is sent - and it will stay visible in the client until it is restarted.

### Example:
```java
player.setTabHeader(
    new ComponentBuilder("BungeeCord").color(ChatColor.RED).bold(true)
        .append("\n  by md_5 and thinkofdeath  ").bold(false).color(ChatColor.GREEN).create(),
    null // Empty footer
);

// "Clear" it after 20 seconds for example
player.resetTabHeader();
```