Add chat position API.

Adds a chat position API so messages can be also sent to the action bar. Currently the messages for the action bar are converted to legacy format so the colors are displayed there too. I just hope Mojang will fix this in the future so we can remove this without having plugin authors change their code.

### Example:
```java
player.sendMessage( ChatPosition.ACTION_BAR,
    new ComponentBuilder( "Action bar message" ).color( ChatColor.GREEN ).create() );
```
Maybe the enum should be renamed as SYSTEM and CHAT appear at the same location; ChatMessageType would be better here i think.