THE USE OF CUSTOM BUILDS
In this thread it is said that you must visit the link but ... how do you prepare to use it with C2 ?.
Or do not you have to do anything ?, just download it and that's it?
Generally speaking yes, just download it and you're done. The "raw" builds provided by the NWjs team are bloated though, that's why I always recommend to use Ashley's clean builds instead. Ashley mostly releases new builds 1-3 days after the official one, I honestly think it's worth the wait.
Another thing, I read that you will be able to have some tabs open in the next version 0.40 or 0.41.
Does this mean that I could have the inventory in one tab and the game in another?
Based on what the NWjs team said, there should be no changes to code necessary. Meaning that whatever JS is used right now, will work the same. I didn't test it out yet since there is no manual page for it, so I don't know how it will work or look like in full detail.
If you want to see what's possible, just download Construct 3 Desktop and check it out. I assume it's all more general JS stuff and less NWjs specific.