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:

After, normal-sized icons:

Before: small icons:

After, small icons:

Reviewers: hein, Plasma

Reviewed By: hein, Plasma

Subscribers: plasma-devel

Tags: Plasma

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