Add panel identifier to panels_node_legacy

As per discussion on IRC, here's a trivial patch to add an identifier field to panels_node_legacy.module. I searched for an existing issue about this, and I found these:
#206317: Adding nodes?
#209491: Display editor shouldn't use theme('panels_pane')
#215896: Forcing node templates on content regions

Since none of them seemed directly about this limitation, I figured it was cleanest to just create a new issue.

p.s. Turns out I'm not going to use panels_node_legacy after-all, it was just easier for my needs to do everything via custom code so it works exactly how I need it, and hook_panels_content_types() gives me everything I need...