Able to save SceneDescription as .p2 file.

[TzuChieh]

First step would be able to save a loaded scene and renders it without problems.

Future things to consider:

• Are comments in scene file kept?
• Should we kept resource names?
• Should we support update existing scene files?

[TzuChieh]

We can save scene descriptions now. be0c1169637c2ed7d703aa2583d045ff77a3162d We may need a new syntax for keeping comments in .p2 file (doxygen keeps ## comments during config update, maybe we can do something similar like keeping //! comments).