Carefully manage save/restore of point in `notmuch-wash-toggle-invisible-action'.

Before the change, save-excursion was used to save the point.  But the
marker saved by save-excursion was inside a region that was deleted,
so that approach is unreliable, (leading to point jumping to a new
position past the button). This patch instead saves point in an
integer variable, and when restoring, carefully avoids moving point
past the button, (in case the new button label is shorter than the old
button label).