Information Panel: Make icons as big as previews

Summary:
Make icons match the preview size.
It prevents a user that had a wide information panel,
seeing a size difference between a preview and an icon despite they can look similar (i.e folder preview).

Test Plan:
Before:
{F8248373, size=full}

After:
{F8248370, size=full}

Reviewers: #dolphin, ngraham, elvisangelaccio

Reviewed By: #dolphin, ngraham, elvisangelaccio

Subscribers: kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D29002