Added AAPTR_PLAYER_GETFLOATYICON constant for retrieving a player's floaty icon

I added the AAPTR_PLAYER_GETFLOATYICON constant so that modders have ways of accessing and manipulating the floaty icon that appears over a player's head. Here's a minimal example WAD that prints a message on the screen when there's an icon over the player's head: Floaty.wad

A feature like this can be useful for modders that want to change how the floaty icon appears under various conditions (e.g. they might want to hide the icon for a particular map or game mode, or when the player's dead, etc.).