Partial fix for bug 401656

Summary:
Only Default Tool should connect to the selectedShapesProxy::selectionChanged,
otherwise arrange actions (both in context menu and arrange docker) get disabled
errorneusly.

This fixes just one part of the bug. The issue with disabled arrange
actions after layer duplication is still present.

CCBUG: 401656

Reviewers: Krita, rempt

Reviewed By: Krita, rempt

Tags: Krita

Differential Revision: https://phabricator.kde.org/D17665