Implemented several improvements to SceneGraph

[lvcabral]

• Added roSGScreen and roSGScreenEvent
• Added support to scripts to be embedded on XML files
• Added roMessagePort (still no support for control)
• Added global functions Sleep() and Wait()

With this PR brs now can run the Roku Hello World, on the root of the sample repository execute:

brs source/source/Main.brs --root ./source

The app will not terminate automatically as it is waiting for an event, so terminate with Ctrl+C, you can add print statements on the Scene init function and see it working.