Fix DND on Folder View containments with locked widgets.

This is a regression from 92665578

, which nerfed DND on the
locked containment, not taking file DND in folder mode into
account. The goal was actually to ignore dropped tabs from
Mozilla Firefox -- this preserves that accomplishment by
only letting through things Folder View would actually care
about.

Reviewed-by: David Rosca <nowrep@gmail.com>
BUG:360578