Moving the selection should reset the buffer

[pjasiun]

1. open http://tests.ckeditor.dev:1030/build/amd/tests/undo/manual/undo
2. type "aaa" in one place,
3. move selection to another,
4. type "bbb",
5. move selection to another,
6. type "ccc",
7. Undo.

Expected: there should be 3 undo steps.

Actual: there is only one undo step.

Changing the selection should reset the buffer. That's it.

[Reinmar]

ckeditor/ckeditor5-typing#21 is another case to cover when fixing this issue.

[Reinmar]

This is very tricky, because we need to reset the buffer when selection changes... but not when that change is caused by typing. So the typing feature will need to block change buffer when they operate on the model.

[Reinmar]

Let's try this way:

1. We add ChangeBuffer.lock() and unlock() methods. They should make the change buffer ignore everything that happens around.
2. When the input or delete feature makes it changes, it should lock the buffer, so it isn't reset by those.
3. Change buffer should also listen to change:attribute and change:range on the selection to reset itself when those things change. But only when it's not locked, ofc.

[maxbarnas]

Prototype of fix for ckeditor/ckeditor5-typing#21 seems to help with this issue. In the meanwhile I'll add tests for the ticket.

[maxbarnas]

Manual test and early version of integration test were just commited. Also, because this issue is tightly related to ckeditor/ckeditor5-typing#21 , there will be testing of styles (Bold and/or Italic).

There are obvious deficiencies e.g., lack of InputCommand, Bold and/or Italic. It'll be corrected soon.