import/export CSV and JSON files compatible with Lego Pick a Brick

[mgodineau]

This PR adds import and export options to work with CSV and Json files. These files are compatible with the online Lego Pick a brick service. This allows to order a whole set of parts there easily.

[ZZJHONS]

How it handles items that have multiple PCCs for a color of an item? Do you add each as a new line in the exported document? Lego PaB not always uses the highest one.

[mgodineau]

When exporting, I just pick the first pcc in the list that matches the desired color. I admit I haven't given much more though to it. I just naively assumed that PaB would recognise every pcc, as long as the part is available.

[mgodineau]

I addressed the pcc issue. Now I make requests to the lego pab website to check which pcc are available, all during csv and json export. Even tho it works, the whole process is now pretty slow, and there is still room for optimisation.