Insert Related Database

[Steffen]

I clearly favor normal LDR or MPD files here because then a user can easily
add part relations to it using the existing editors.
Additionally, the parts can be positioned relatively to another this way, which is nice for
windows+glasses (or: +hinges), wheels+tyres, etc.

I see something nice forthcoming here, especially because of the synergy between Ben and Sergio, NICE!