Give file and folder names a bit more room in Folder View

Summary:
Slightly decrease the side margins within Folder View delegates, to give titles a tiny bit more room. This very slightly improves things for people who have desktop files with multi-line titles.

CCBUG: 379432

Test Plan:
Before, normal-sized icons:
{F5755097}

After, normal-sized icons:
{F5755128}

Before: small icons:
{F5755102}

After, small icons:
{F5755127}

Reviewers: hein, #plasma

Reviewed By: hein, #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11358