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/default/README.txt

@@ -123,6 +123,8 @@ paramat (CC BY-SA 3.0):
   default_fence_rail_junglewood
   default_fence_rail_pine_wood
   default_fence_rail_wood -- Derived from a texture by BlockMen (CC BY-SA 3.0)
+  gui_hotbar.png
+  gui_hotbar_selected.png
 
 TumeniNodes (CC BY-SA 3.0):
   default_desert_cobble.png -- Derived from a texture by brunob.santos (CC BY-SA 3.0)
@@ -150,7 +152,10 @@ BlockMen (CC BY-SA 3.0):
   default_mineral_mese.png
   default_meselamp.png
   bubble.png
-  gui_*.png
+  gui_formbg.png
+  gui_furnace_arrow_bg.png
+  gui_furnace_arrow_fg.png
+  gui_hb_bg.png
 
 sofar (CC BY-SA 3.0):
   default_aspen_sapling
@@ -330,6 +335,10 @@ http://freesound.org/people/Ryding/sounds/94337/
 Ferk (CC0 1.0):
   default_item_smoke.ogg, based on a sound by http://opengameart.org/users/bart
 
+sonictechtonic (CC BY 3.0):
+https://www.freesound.org/people/sonictechtonic/sounds/241872/
+  player_damage.ogg
+
 
 Models
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