Remove row in complex tables sometimes provides invalid results

[jodator]

Extracted from: ckeditor/ckeditor5-table#286 (comment)

📝 Provide detailed reproduction steps (if any)

Sometimes when the table has complex geometry and you remove a row ( with multiselect or not ), cells are changing their position in table:

✔️ Expected result

The table geometry should be preserved (ie the cell should stay on the same column)

❌ Actual result

It is randomly pushed in weird positions.

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[jodator]

This also look like a case for #6439 - to check.

[jodator]

The algorithm is wrong 🤷 The order of operations isn't OK. Below is an idea of how this should work:

Steps should be:

1. Cache all cells that spans over removed rows.
2. Move all cells from removed rows (and reduce their rowspan) to a row after removed rows. (work on cached table structure from TableWalker).
3. Remove rows.
4. Trim cached cells from step 1.

Bonus: fewer operations in multi-row removal.