Clip the flickable parent item

Summary:
Without clipping the contents, they can draw stuff outside of the scrollview
bounds, leading to issues with content being drawn underneath the scrollbar,
among others.

BUG: 416877

Test Plan:
Before:

After:

Reviewers: Kirigami, ngraham, mart

Reviewed By: Kirigami, ngraham, mart

Subscribers: ngraham, apol, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D27027