Add "veryLongDuration" animation duration

Summary: As discussed in D28033#630649.

Test Plan: Compiles. Not used anywhere yet, so no testing possible.

Reviewers: VDG, Kirigami, mart

Reviewed By: VDG, Kirigami, mart

Subscribers: davidre, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D28143