Simply range walking codes

[Garry Yao]

The new range walker API is about to be ready with #3367, it introduced a completely new simplified way to perform daily range work, it's concise and powerful to support the the following ways when walk along range.
We should try to replace the bells and whistles we've had in v2 for ranges manipulation with this new API after we close #3367.

Now we could:

1. Walking along the content inside the range, mostly useful for style system.

   outside [->...inside...<-]outside 
   

   With

   var walker = new CKEDITOR.dom.walker( range ), node;
   while ( node = walker.next() )
   {
           // Node processing...
   }
   

2. Walking along the other opposite two sides of the range, mostly useful for enlargement and checking block boundary.

   outside <-[ ...inside...]-> outside 
   

   With

   // E.g. Walking along the left out side 
   var walkerRange = new CKEDITOR.dom.range();
   walkerRange.setStartAt( document.getBody(), CKEDITOR.POSITION_AFTER_START );
   walkerRange.setEndAt( range.startContainer, range.startOffset );
   walker.guard = function()
   {
           //// Node processing...
   };
   do
   {
           node = walker.next()
   }
   while (  node )
   
   

3. Retrieve the boundary nodes of collapsed range:

   leftBoundaryNode<-^->rightBoundaryNode
   

   With

   // E.g. Get the end boundary node after the range.
   var walkerRange = new CKEDITOR.dom.range();
   walkerRange.setStartAt( range.endContainer, range.endOffset );
   walkerRange.setEndAt( document.getBody(), CKEDITOR.POSITION_BEFORE_END );
   var endBoundaryNode = walkerRange.next();
   

4. Even other usages could be expanded...