Selections are not preserved when undoing a cut()/delete()

If you undo a cut() the selection is not preserved. Editors on Windows and Mac preserve the selection. See autotest QTextEdit::copyAvailable case 7.

If you undo a cut() the selection should be in the state before the cut and a copyAvailable signal should be emitted. With the current QTextEdit the selection is not set back after the undo and no signal is emitted.

Another case where this does not work, is when selecting some text, entering some characters (selected text is removed) and undoing again. The original selection is lost.

See change: 238164