Add undeclared Class Properties

Fixes Creation of dynamic property _HWLDF_WordAccumulator::$_lines /
$_line / $_group / $_tag is deprecated in ./inc/DifferenceEngine.php
on line 936

And proactively caught another bunch in _Diff3_Op class (not reported
in unit tests)

inc/DifferenceEngine.php

@@ -929,6 +929,18 @@ static function css($classname){
 
 class _HWLDF_WordAccumulator {
 
+    /** @var array */
+    protected $_lines;
+
+    /** @var string */
+    protected $_line;
+
+    /** @var string */
+    protected $_group;
+
+    /** @var string */
+    protected $_tag;
+
     function __construct() {
         $this->_lines = array();
         $this->_line = '';
@@ -1439,6 +1451,18 @@ function _diff3($edits1, $edits2) {
  */
 class _Diff3_Op {
 
+    /** @var array|mixed */
+    protected $orig;
+
+    /** @var array|mixed */
+    protected $final1;
+
+    /** @var array|mixed */
+    protected $final2;
+
+    /** @var array|mixed|false */
+    protected $_merged;
+
     function __construct($orig = false, $final1 = false, $final2 = false) {
         $this->orig = $orig ? $orig : array();
         $this->final1 = $final1 ? $final1 : array();