Move hotbar code/textures, and damage sound, from player_api to default

These are unrelated to the Player API and player object.
All other GUI/HUD code and textures are in default.
All other engine hardcoded sounds are in default.
The player_api mod is highly likely to be used unchanged in new games,
so logical grouping of content will help the creation of new games.

mods/player_api/README.txt

@@ -4,6 +4,7 @@ See license.txt for license information.
 
 Provides an API to allow multiple mods to set player models and textures.
 Also sets the default model, texture, and player flags.
+This mod is only for content related to the Player API and the player object.
 
 Authors of source code
 ----------------------
@@ -19,14 +20,6 @@ stujones11 (CC BY-SA 3.0):
 Jordach (CC BY-SA 3.0):
   character.png
 
-paramat (CC BY-SA 3.0):
-  gui_hotbar.png
-  gui_hotbar_selected.png
-
 celeron55, Perttu Ahola <celeron55@gmail.com> (CC BY-SA 3.0):
   player.png
   player_back.png
-
-sonictechtonic (CC BY 3.0):
-https://www.freesound.org/people/sonictechtonic/sounds/241872/
-  player_damage.ogg