Client-side implementation of PlayerEvent.StartTracking and .StopTracking

    net.minecraftforge.event.entity.player.PlayerEvent.StartTracking
    net.minecraftforge.event.entity.player.PlayerEvent.StopTracking

The events should trigger if an Entity is added to or removed from a client's world. This happens for example when the player enters or leaves the tracking radius of an Entity.

At the moment there is no client-side implementation for this event.

**PlayerEvent.StartTracking can be generated in:**
`net.minecraft.client.multiplayer.WorldClient.addEntityToWorld(int, Entity)`
It is called when one of the following packets is handled by the client:

    S0CPacketSpawnPlayer
    S0EPacketSpawnObject
    S0FPacketSpawnMob
    S10PacketSpawnPainting
    S11PacketSpawnExperienceOrb

**PlayerEvent.StopTracking can be generated in:**
`net.minecraft.client.multiplayer.WorldClient.removeEntityFromWorld(int)`
It is called when one of the following packets is handled by the client:

    S0DPacketCollectItem
    S13PacketDestroyEntities

**I have tested this PR using the following test mod:**
https://gist.github.com/nairol/acea686439944e58c0f7