Introduce an adoption command in the wallpaper knsrc file

Summary:
Using the same semantics as the InstallationCommand, makes it possible for the
client to get a kns resource adopted.

Needs further standarization, I have implemented this on Discover and seems
to work just fine.

Reviewers: whiting, leinir, #plasma, mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4307