Ensure that GlobalDrawer topContent always stays on top

Summary:
Right now, if you define topContent for your GlobalDrawer, it's added to the scrollview
so it can actually scroll out of sight, making it not always on top.

This patch moves the topContent above the scrollview so it always stays on top.

Test Plan:
Discover's GlobalDrawer toolbar no longer strangely scrolls out of view:

No regressions in Discover when the view is not scrollable

No regressions that I could detect in Kirigami Gallery, though this probably needs lots
more testing to ensure that nothing has exploded

BUG: 389533
FIXED-IN: 5.64

Reviewers: Kirigami, mart, apol, ahiemstra

Reviewed By: ahiemstra

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D25019