Promote Xcode resources back to source code

[rauanmayemir]

After #68, dune maintains all the configs and xcode generation. The good thing is that now we don't need to care about different configs and make steps, dune smoothly takes care of all that and as a bonus, it all works in watch mode.

The not so good thing is that Xcode project cannot be promoted back to source code because it's a directory, and even if it can be, xcode will point to resources (images, fonts) in the dune's build dir (they're copied during build like anything else), not the original source dir.

This hits the DX as you can't open the xcode project, add your assets, and have it all reconciled with dune.

I'm not sure how to properly solve this, maybe we should be okay with people opening XCode from a generated _build dir, but promote all the added assets back to project sources.