Added support for the indeterminate state as it is supported on OS X.

[hendriks73]

On OS X an indeterminate state is supported for JCheckBox. Instead of the check mark, it's a simple horizontal line.

It would be nice, if that was supported by Darcula as well.

See Radar #8930094: https://developer.apple.com/library/content/releasenotes/Java/JavaSnowLeopardUpdate4LeopardUpdate9RN/ResolvedIssues/ResolvedIssues.html