Pixel align children of GridViewInternal

Summary:
The scroll bar size can be odd (for breeze it's 21), which causes leftMargin
to be 12.5. This causes every delegate inside to be blurred.

Test Plan: Monkeypatched, now kcm_style is no longer blurred.

Reviewers: Frameworks, broulik, mart, davidedmundson

Reviewed By: broulik, davidedmundson

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D29503