D25311: Add KColumnHeadersProxyModel

[Arjen Hiemstra]

ahiemstra marked an inline comment as done.

REPOSITORY
  R275 KItemModels

REVISION DETAIL
  https://phabricator.kde.org/D25311

To: ahiemstra
Cc: kossebau, vkrause, davidedmundson, kde-frameworks-devel, LeGast00n, GB_2, 
michaelh, ngraham, bruns