T/1053 Incorrectly set context.forceWeakRemove for MergeDelta during OT

[scofalik]

Suggested merge commit message (convention)

Fixed: Fixed bug when additional list item has been created when undoing setting block quote on list followed by splitting list item in that list. Closes ckeditor/ckeditor5#4132.

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Additional information

1. When working on this issue I discovered another one. RemoveOperation x MoveOperation was changing context.isStrong value directly, which affected further transformations. Now context.isStrong is assigned to a different variable and that variable is changed if needed.

2. Tests for this issue will be written in ckeditor5-undo.

[scofalik]

Tests: ckeditor/ckeditor5-undo#69

[Reinmar]

Tests for this change need to also be in this package. There can be additional integration tests in ckeditor5-undo but we need some basic verification here. Otherwise, someone (most likely you) will complain that he changed something in ckeditor5-engine, tests passed, but they failed in ckeditor5-undo ;).

[scofalik]

I will write the test but you have to be aware that these kinds of tests are difficult to write in "non-integrational" way.

It's even hard to name a test case because these are all "private" switches and flags of closed and complicated process.

[Reinmar]

But I didn't say that the test must be non-integrational. I just want it to be here. And there's a whole range of approaches and API levels between tests in the undo package and unit tests (which we don't want). In other words – I'd like to see as high-level test as you could implement in here.

[scofalik]

I know I know :). I just wanted to explain why I haven't written any test in the first place.

[scofalik]

I've added additional test (and also found another conceptual problem - with how MoveOperations are broken into multiple operations - and fixed it).