D26898: expose an id for the wallpaper

[Nathaniel Graham]

ngraham added a task: T10495: wallpaper cache.

REPOSITORY
  R242 Plasma Framework (Library)

REVISION DETAIL
  https://phabricator.kde.org/D26898

To: mart, #plasma
Cc: kde-frameworks-devel, LeGast00n, cblack, michaelh, ngraham, bruns