Reversed ReinsertOperation should remove nodes back to the same graveyard holder

[scofalik]

Whenever new RemoveOperation is created, $graveyard root is checked for how many $graveyardHolder nodes it has and the target of RemoveOperation is set to the new $graveyardHolder that will be inserted during operation execution at the end of $graveyard.

Simple example of situation when editor throws. Consider a batch that removes a character and removes bold from it afterwards. It's based on real scenario where post fixer callback fixes model and adds it's changes to the same batch (original issue: https://github.com/ckeditor/ckeditor5-list/issues/51):

1. REMOVE from $root [ 0, 1 ], 1 node, to $graveyard [ 0, 0 ].
2. REMOVE ATTRIBUTE "bold" from $graveyard [ 0, 0 ] - [ 0, 1 ].

When it is undone, following batch is generated:

1. SET ATTRIBUTE "bold"=true on $graveyard [ 0, 0 ] - [ 0, 1 ].
2. REINSERT from $graveyard [ 0, 0 ], 1 node, to $root [ 0, 1 ].

Now redo:

1. REMOVE from $root [ 0, 1 ], 1 node, to $graveyard [ 1, 0 ] (new remove operation, creates new graveyard holder).
2. REMOVE ATTRIBUTE "bold" from $graveyard [ 0, 0 ] - [ 0, 1 ]. <-- this throws, as there are no nodes in [ 0, 0 ].

It makes sense that RemoveOperation created from ReinsertOperation reversing should put nodes back in their original place. The only scenario where ReinsertOperation is reversed is redo.