Fix: activate "next/previous layer" shortcut for layers or group which are invisible.

Summary: use PgDn, PgUp as shortcuts to move through the invisible layers inside a group.

Test Plan: Checked by building it.

Reviewers: rempt, dkazakov

Subscribers: dkazakov, Bollebib, woltherav, Krita

Tags: Krita

Differential Revision: https://phabricator.kde.org/D4373