Selecting cells in a QTextTable is buggy

When using either the mouse or cursor keys to select cells in a table, it often ends up wrong.

Try selecting a cell in the middle of a table and then holding down the mouse button move around this cell. To the northwest, and south east it works as expected, but moving to the norteast and southwest directions from the original cell, will see that you are missing a whole column of cells in the selection.

Another related bug is (with an empty table) placing the cursor in a cell and then going right by shift+right two times. This correctly selects 3 cells in total. Now try and go to the left once (still holding down the shift key). On one click you skip a cell and are back in the original cell.

You can use the qtdemo of textedit to try out thes things.

I've looked at the code, and found a solution and provided unittests. see merge request 2541