Merge pull request #36 from CroniD/master

LinuxContentDesign & MacOsContentDesign SystemColor Usage is wrong #34
DistinctColumnsItems can properly handle null values, so use the TreeSet directly instead of converting between collection types.
Merge pull request #38 from ThePumpingLemma/master

DistinctColumnsItems can properly handle null values, so use the TreeSet...
Merge pull request #1 from eugener/master

Update code base