Link together Atlas projects in XCode

[historic_bruno]

Using our new premake4 build system on OS X, the generated XCode projects are all separate: the pyrogenesis project that builds the game, AtlasUI that builds the AtlasUI library, ActorEditor that builds the actor editor frontend, and Collada. You can open them all at once in XCode, but they build separately, so it's not the most efficient workflow.

These projects should be combined as they are for MSVC. That way if someone runs updates-workspaces with the --atlas flag then AtlasUI, AtlasObject, AtlasScript, and ActorEditor projects should all appear in the single XCode window with the others, with properly linked dependencies.