Embed assets inside executables on desktop

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 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.