Add footsteps for water - swimming sounds.

This is a redo of #436.

I've taken a single, longer audio sample and extracted 3 samples
of 2.0 length, and gave them all a 0.5 sec fade-in and fade-out.

I added a 4th sample that's essentially a nearly-quiet version of
one of the prior sounds.

This combined with the rate of footstep sounds results in a
randomly changing underwater sound that blends in and out somewhat
nicely.

Code entirely by @MirceaKitsune.

mods/default/README.txt

@@ -221,3 +221,9 @@ Mito551 (sounds) (CC BY-SA):
   default_dirt_footstep.1.ogg
   default_dirt_footstep.2.ogg
   default_glass_footstep.ogg
+
+Water footsteps - CC-BY-3.0 - by akemov - http://www.freesound.org/people/akemov/sounds/255597/
+  default_water_footstep.1.ogg
+  default_water_footstep.2.ogg
+  default_water_footstep.3.ogg
+  default_water_footstep.4.ogg

mods/default/functions.lua

@@ -83,6 +83,20 @@ function default.node_sound_glass_defaults(table)
 	return table
 end
 
+function default.node_sound_water_defaults(table)
+	table = table or {}
+	table.footstep = table.footstep or
+			{name="default_water_footstep", gain=0.25}
+	table.dug = table.dug or
+			{name="default_water_footstep", gain=0.4}
+	table.dig = table.dig or
+			{name="default_water_footstep", gain=0.4}
+	table.place = table.place or
+			{name="default_water_footstep", gain=0.4}
+	default.node_sound_defaults(table)
+	return table
+end
+
 
 --
 -- Lavacooling

mods/default/nodes.lua

@@ -1092,6 +1092,7 @@ minetest.register_node("default:water_source", {
 	liquid_viscosity = 1,
 	post_effect_color = {a = 103, r = 30, g = 60, b = 90},
 	groups = {water = 3, liquid = 3, puts_out_fire = 1},
+	sounds = default.node_sound_water_defaults(),
 })
 
 minetest.register_node("default:water_flowing", {
@@ -1137,6 +1138,7 @@ minetest.register_node("default:water_flowing", {
 	post_effect_color = {a = 103, r = 30, g = 60, b = 90},
 	groups = {water = 3, liquid = 3, puts_out_fire = 1,
 		not_in_creative_inventory = 1},
+	sounds = default.node_sound_water_defaults(),
 })
 
 
@@ -1183,6 +1185,7 @@ minetest.register_node("default:river_water_source", {
 	liquid_range = 2,
 	post_effect_color = {a = 103, r = 30, g = 76, b = 90},
 	groups = {water = 3, liquid = 3, puts_out_fire = 1},
+	sounds = default.node_sound_water_defaults(),
 })
 
 minetest.register_node("default:river_water_flowing", {
@@ -1230,6 +1233,7 @@ minetest.register_node("default:river_water_flowing", {
 	post_effect_color = {a = 103, r = 30, g = 76, b = 90},
 	groups = {water = 3, liquid = 3, puts_out_fire = 1,
 		not_in_creative_inventory = 1},
+	sounds = default.node_sound_water_defaults(),
 })