Fix crash when selecting non-visible objects

The selection describer code was assuming that a selection can only hold visible items (SPItems) but it actually holds SPObjects, which can include non-visible items.

This code changes the selection describer to more consistently work with SPItems only, and to ignore SPObjects.

Fixes #6125 (closed)

I tested that the crash no longer happens (but that you can still see the XML selection happening if you have the Edit -> XML dialog open).