introduce candidateContainments

Summary:
DesktopView contains a property candidateContainments which are all containments with the same screen for all activities.
Plasma mobile can use it to do activity switching with a swipe gesture
replaces D11333

Test Plan:
tested with gesture based activity switching in plasma mobile including creating and deleting
of activities

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D12351