slideshow/pict: add `#:alpha' argument to `pin-line'
fix mark handling for impersonatored procedures to match docs

Using an impersonator property to represent an application mark was
a bad choice. The property gets propagated, so it is also on any later
chaperone layer, and then things go bad: the docs say that special
treatment is triggered by supplying an argument to `impersonate-property',
but it was actually triggered by the chaperone having the property.

Change the implementation to match the documentation. Using an
impersonator property to supply the mark should be regarded as a hack,
but now the implementaiton is at least consistent with the documentaiton.