Correct margin for left sidebar

Summary:
Sidebar content too close to the left boundary of the window.

This patch adds left margin to some of the sidebar contents.

BUG: 381535

Test Plan:
Before:
{F5524922}
After:
{F5524923}

Reviewers: #kde_applications, ngraham, rkflx

Reviewed By: ngraham, rkflx

Subscribers: #kde_applications

Tags: #kde_applications

Differential Revision: https://phabricator.kde.org/D9145