SDL versioning and version converting scripts

[TzuChieh]

This feature should be done as soon as possible, since every change in the SDL syntax can potentially break a bunch of existing scene files.

[TzuChieh]

The syntax will be implemented as magic comments, as they generally do not need to be parsed during scene loading. Other meta information can be added in the same way as version info.

The syntax:

//: <key1>=<value1>
//: <key2>=<value2>
...