Automatic asset embedding

When embed=true for assets in project.xml, this will use @:bitmap, @:sound, etc. to embed them into the executable on Windows/Mac/Linux instead of copying them to the bin directory. When embed=false, the old behavior of copying to the bin directory is preserved. This will allow distribution on desktop as a single file, without leaving assets and data files exposed.

This probably shouldn't be merged as is, because embed=true by default so a lot of people will see a sudden and unexpected behavior change. Just wanted to submit it for discussion to get the ball rolling. One option for dealing with this would be to make embed=false by default on targets where no embedding was previously being done, but true on Flash to preserve the current default behavior. (As far as I know, the embed parameter doesn't actually do anything on most targets currently - correct me if I'm wrong.)