Fix selection at borders of insets (#9487)

Fix selection at borders of insets (#9487)

When at the last position in an inset, selecting to the right
should select the entire inset. This only worked if there was
already a selection (i.e. the selection was started not at the
boundary).

The behavior of this bug was changed by commit 73a7bf9d.  Before
that commit, if at the last position of an inset you select to
the right, nothing is selected but the selection is set. If you
select once more to the right, because the selection is set the
needsUpdate condition is satisfied so the whole inset is selected.

Note that everything here applies also to "first position of an
inset" and selecting to the left. By "selecting", I am referring
to LFUN_{CHAR,WORD}_{FORWARD,BACKWARD}_SELECT.