Fix overlap of the first item in KFilePlacesView

Summary:
The first item overlapped both the section header and the item
below. This was caused by taking the spacing wrongly into account.
Additionally, the paint function's option's rect was not moved correctly,
as the unmodified option.rect was used in some places.

Test Plan: https://i.imgur.com/LJuACt2.png -> https://i.imgur.com/nYpdoXn.png

Reviewers: Frameworks, dfaure, renatoo

Reviewed By: renatoo

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D9863