Adjust formula for lateral area's icon size

Summary:
Use 72 denominator when calculating icon size; this lets up cover a better range of hi-DPI screens with the
correct size icon (48px)

Test Plan:
before: (32 px @162 DPI)

after: (48px @162 DPI)

Reviewers: tcanabrava, patrickelectric, laysrodrigues

Reviewed By: tcanabrava, patrickelectric, laysrodrigues

Subscribers: Atelier: General Development

Tags: Atelier: General Development

Differential Revision: https://phabricator.kde.org/D16855