always sort contacts alphabetically

- introduce pairsByKeys() (from http://www.lua.org/pil/19.3.html but adding current index and total length as iterator return values)
- use pairsByKeys() everywhere, where contacts are iterated over (for now only when displaying in gui.lua) so the order stays the same although the structure is a hashmap
- correctly update selected position when adding/removing contacts to TO/CC/BCC fields
- remove print statements in storage.lua

storage.lua

@@ -36,6 +36,25 @@ mail.getContacts = function(playername)
 	return mail.read_json_file(mail.getContactsFile(playername))
 end
 
+function pairsByKeys(t, f)
+	-- http://www.lua.org/pil/19.3.html
+	local a = {}
+	for n in pairs(t) do table.insert(a, n) end
+	table.sort(a, f)
+	local i = 0		-- iterator variable
+	local iter = function()		-- iterator function
+		i = i + 1
+		if a[i] == nil then
+			return nil
+		else
+			--return a[i], t[a[i]]
+			-- add the current position and the length for convenience
+			return a[i], t[a[i]], i, #a
+		end
+	end
+	return iter
+end
+
 mail.setContacts = function(playername, contacts)
 	if mail.write_json_file(mail.getContactsFile(playername), contacts) then
 		return true
@@ -51,7 +70,6 @@ function mail.read_json_file(path)
 	local content = {}
 	if file then
 		local json = file:read("*a")
-		print(string.format('read from %s:  %s', path, json))
 		content = minetest.parse_json(json or "[]") or {}
 		file:close()
 	end
@@ -61,7 +79,6 @@ end
 function mail.write_json_file(path, content)
 	local file = io.open(path,"w")
 	local json = minetest.write_json(content)
-	print(string.format('writing to %s:  %s', path, json))
 	if file and file:write(json) and file:close() then
 		return true
 	else