D26457: Introduce shadow API

[Vlad Zahorodnii]

zzag added a task: T12496: Shadows API in KWindowSystem.

REPOSITORY
  R278 KWindowSystem

REVISION DETAIL
  https://phabricator.kde.org/D26457

To: zzag, #kwin
Cc: kde-frameworks-devel, LeGast00n, GB_2, michaelh, ngraham, bruns