feat: Column spans

[Gustl22]

Add's the colspan feature for table rows.

Closes #989, #896, #645, #313, #256, #167

[Gustl22]

IntrinsicColumnWidth is not yet tested. Also I didn't get the whole concept, so there might be some edge cases, which I haven't considered (especially in the paint method, where _widths are consumed).

[DavBfr]

Can you add a test?

[Gustl22]

Sure, I just have limited resources and vacation soon, so if someone wants to finish up, I don't mind.
I'll get back to it in a few weeks.

[Gustl22]

I added a test, but it does not precisely check e.g. the different box properties.
If this should be also done, let me know and how to best approach it.

[Gustl22]

I have a small problem: the tests now complain, that there's a difference (which is indeed in the renders).
But if I inspect this in a pdf viewer / firefox / chrome etc. There isn't any difference visisble:

I suspect the test renderer interprets this as small gap and therefore outputs the mean as grey?
widgets-table.pdf

Should we ignore this or solve it another way?

[DavBfr]

Yes, that's possible. Seems to be at the intersection of each cell.

[Gustl22]

To allow col spans the border must be calculated for each cell and not for the whole width / height of the table any more. So there now exist infinite small fake gaps, which in theory should not be visible, and also aren't in my inspection in the pdf readers, just the pdftocairo does this I think...

[DavBfr]

That's fine then.

[Gustl22]

I now wonder, if it would make sense to introduce a new widget, like TableCell where one can define the col and row span. Otherwise I don't see a good way to support row spans in the future. Also this would remove potential errors in defining the col spans on the Row level.

See also:
flutter/packages#5917