Handle Shift+arrows inside table cells

[Reinmar]

While reviewing selection post-fixer I found out that the biggest problem with it is how it fixes selection made by Shift+arrow if it started inside a table cell.

The expected result is that once you reach a cell's limit we either:

• Trim the selection to that cell only.
• Select the starting table cell and the one where you moved the caret (which requires https://github.com/ckeditor/ckeditor5-table/issues/63).

I described how we can implement it in ckeditor/ckeditor5-engine#1431 (comment).

[Reinmar]

There's a more general ticket about the same behaviours in widgets: https://github.com/ckeditor/ckeditor5-widget/issues/42.