Removing the Activity wrapper class from the Shell

Summary:
After the previous patch which removed the unused
Activity class related events, it started becoming clear that
said class is only really used to store the name of the
containment plugin that should be loaded for an activity.

Instead of keeping a map of these objects (QObjects) with
all their baggage, the shells now only keep the map of
activity IDs and corresponding plugin names.

Reviewers: mart, bshah, davidedmundson, Plasma

Reviewed By: davidedmundson, Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2176