Add an overlay component for item activity feedback

Summary:
This is the very first steps toward allowing for better, more involved
feedback on what is happening with the items (such as download progress,
the ability to abort installations, and such things).

BUG:413441

Also removes a now unneeded passive notification call

Test Plan:
Without this patch, as mentioned in https://bugs.kde.org/show_bug.cgi?id=413441

With this patch:

Reviewers: KNewStuff, Frameworks, Plasma, ngraham

Reviewed By: ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25189