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
 ------

mods/default/init.lua

@@ -14,10 +14,10 @@ default.get_translator = S
 
 -- GUI related stuff
 minetest.register_on_joinplayer(function(player)
+	-- Set formspec prepend
 	local formspec = [[
 			bgcolor[#080808BB;true]
 			listcolors[#00000069;#5A5A5A;#141318;#30434C;#FFF] ]]
-
 	local name = player:get_player_name()
 	local info = minetest.get_player_information(name)
 	if info.formspec_version > 1 then
@@ -26,6 +26,10 @@ minetest.register_on_joinplayer(function(player)
 		formspec = formspec .. "background[5,5;1,1;gui_formbg.png;true]"
 	end
 	player:set_formspec_prepend(formspec)
+
+	-- Set hotbar textures
+	player:hud_set_hotbar_image("gui_hotbar.png")
+	player:hud_set_hotbar_selected_image("gui_hotbar_selected.png")
 end)
 
 function default.get_hotbar_bg(x,y)