Add a consistent mechanism to package applications that build upon Plane

[fwcd]

Currently every Plane implementation requires a different way of embedding custom PlaneApp-based sources into its toolchain. For example:

• ios-bridge has the APP-INFO.h file that describes where the PlaneApp source files are placed relative to the Xcode project
• sdl-cpp requires a specific launch class and also relies on relative paths to the PlaneApp source files

Although some of these constraints might be unavoidable, it would be nice to have a consistent way of packaging and launching custom Plane-based applications.