Make it possible to adopt resources, mostly for system-wide settings

Summary:
Makes it possible to specify commands that will put the resource in use, such
as changing the wallpaper or changing the icon theme or color scheme.

Test Plan: tested with some plasma resources, see patches against plasma-workspace and plasma-desktop.

Reviewers: Frameworks, Plasma, mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D4365