When pulling an assembly from Silo, the linked component files were not
downloaded, causing FreeCAD to report 'Link not restored' errors for
every external reference.
Add _pull_dependencies() that queries the BOM API to discover child
part numbers, then downloads the latest file revision for each child
that doesn't already exist locally. Recurses into sub-assemblies.
Silo_Pull.Activated() now calls _pull_dependencies() after downloading
the assembly file and before opening it, so all PropertyXLink paths
resolve correctly.
