Improved iOS launch image usage

Currently Lime generates blank white launch images for missing iOS screen sizes, regardless of platform restrictions (eg. iPad only) or orientation constraints (eg. Landscape only). This means:

* If you have eg. a non-retina launch screen but no retina version defined, you get a white screen during startup rather than letting iOS fall back to the non-retina version.

* Build times when updating XCode projects are slower than they need to be.

These changes improve these issues by selectively only including the launch images required for a given setup in the project file (removing the need to generate the placeholder PNGs), resulting in no more white splash screens and bringing build times (for "lime update ios") on my Macbook Air from 10.4s down to 2.3s.