[REVIEW ONLY] Project manager revamp

Note that this pull request is based on jeff/splitview-1x2. I'll make a new PR once that is merged into master.

cc @ingorichter 
The comment of `FileTreeViewModel.prototype.isFilePathVisible` states that the return value can be either `boolean` or `null`, but here is a case where we return an empty Array. Will this path ever be executed?