Allow iOS asset catalogs

[teobugslayer]

Allow iOS asset catalogs to be added in the app and added to the Xcode project. Also see https://github.com/NativeScript/template-hello-world/issues/22

[e2l3n]

Per our discussion earlier we decided to postpone the implementation of image.xcassets catalogs which would enhance end users.
The current PRs remove the hardcoding of icon fileset references and update the resources destination path.