Setting column as header should split it col-spaned cells

[jodator]

📝 Provide detailed reproduction steps (if any)

1. Merge two or more table cells at the beginning of a row.
2. Set column heading on merged cell.

✔️ Expected result

The behavior should follow the logic for setting heading rows - it should split col-spanned cells which spans extends over the column heading section.

❌ Actual result

The restriction is not applied.

Note that creating an identical table is not possible using merge cells and merge (right/left) cell commands.

Possible solution

For better UX when selection is in cells that are already merged I can see two options:

1. We use it's column and split the merged cell.
2. We set heading over columns that this cell spans.

Case 1: split selected cell	Case 2: Extend heading rows

For case 2: Any other merged cell (apart from selected) should not extend the heading section and should be split.

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[jodator]

Utility for this is created in #6121.