Small Row cleanups

Small Row cleanups

Move declaration of RowList to Row.h

Move initialization of POD members of Row and Row::Element to declaration.

Make method isVirtual() depend on type.

Add new row element type INVALID and method isValid()

Make methods R::E::left/right_pos inline.

Add method R::E::splitAt() that returns an element containing the
remaining stuff, or an invalid element if nothing was split. breakAt
is now a simple wrapper around this function.

Add method R::push_back().