Implements CallFunction with JS::MutableHandle<T> return type.

Changes the CallFunction implementation to use macros because otherwise we'd have to write twice as many functions manually.
Adapts GetSavedGameData to use the new function template. Additional callers will be changed in future commits.

Refs #2415
Refs #2462 


git-svn-id: http://svn.wildfiregames.com/public/ps/trunk@15541 3db68df2-c116-0410-a063-a993310a9797
Adds support for passing JS::HandleValue to SetProperty and CallFunctionVoid without using ugly casing.

Also includes one little "demo-usecase", but additional code will be changed to use that in future commits.

Refs #2415
Refs #2462 


git-svn-id: http://svn.wildfiregames.com/public/ps/trunk@15542 3db68df2-c116-0410-a063-a993310a9797
Merge 'remotes/trunk'