Add support for specifying LaunchScreen.ui.xml

[TadeasKriz]

LaunchScreen.ui.xml would be a special kind of component that would not only generate Swift code, but also a .nib/.storyboard files.

This would let us make InitialController easily without duplication.

Example:

<Component launchScreen="LaunchScreen.xib">
    <ImageView image="backgroundImage" layout:edges="super" />
    <ImageView image="logo" layout:center="super" />
</Component>

We should decide whether to allow all properties we do in normal Component UI XMLs and then ignore some of them when generating the LaunchScreen.xib/storyboard file, or if we disallow those properties directly.