Get rid of lyx::next uses for RandomAccessList

Get rid of lyx::next uses for RandomAccessList

These uses are inefficient (a loop really) and require that pit_type
is ptrdiff_t.

Instead, RandomAccesslist::constIterator is renamed to iterator_at and
a version adding a non-const iterator is added. Additionally, the
method retirns end() when position is equal to the size of the
container (see #11861).

lyx::next and lyx::prev are removed, and std::prev is used in the few
places where the code requires it (for no good reason IMO).