Fix selections in tables that span over several cells

Fix selections in tables that span over several cells

In the code that converts text layouts to subtrees in the scene
graph specifically for TextEdit, there was a cutoff to treat
text tables as single nodes in the graph (for simplicity).
However, this breaks selections, since the ranges spanned by
each cell will be interpreted as overlapping, messing up the
selection merging logic. We need the same approach here as for
any other text frame where we check frame boundaries.

[ChangeLog][TextEdit] Fixed issues with selections that spanned
several cells in a table.

Change-Id: I789041d84b5d163e209488f8f2f1f83a6471389f
Task-number: QTBUG-46928
Reviewed-by: Pierre Rossi <pierre.rossi@theqtcompany.com>