Unbreak presentation functionality in inspector.

SWANK:*INSPECTEE-PARTS* has been gone for some time now, but
contrib/slime-presentations.lisp still used it to check object
availability in the inspector. Hid this logic behind existing
SWANK:INSPECTOR-NTH-PART, which behaves more like GETHASH.

* swank.lisp (inspector-nth-part): Don't break if index out of bounds,
return second FOUNDP value instead.

* contrib/swank-presentations.lisp (lookup-presented-object): No
longer reference *inspectee-parts* removed a long time ago.

* contrib/swank-presentations.el
(slime-menu-choices-for-presentation): slightly friendlier pop-up menu
title, could be friendlier maybe if object type were displayed
instead.