Partial cleanup of the row selection code

Partial cleanup of the row selection code

This is preliminary work, this code still feels too complicated for
its own good.

Let Row::isMarginSelected return false when Row::selection() is false
(the other changes are indentation).

This allows to remove the test for selection() in
setSelectionAndMargins, so that begin/end_margin_sel are always set
correctly.

Add clearSelectionAndMargins() instead of calling directly setSelection
(which is now private) with arguments (-1, -1).

Fixes bug #10972.