[Folder view] Scale selection and preview buttons with item size

Summary:
Right now the selection and preview buttons that appear on hover are very very tiny,
and remain very very tiny even when the icon size is large.

This patch increases the size--and therefore visibility and clickability--of the buttons
as the icon size increases.

BUG: 411073
FIXED-IN: 5.17.0

Test Plan:
No change for icon sizes at slider positions 1, 2, and 3, where items are quite small.

Slider position 4:

Slider position 5:

Slider position 6:

Reviewers: hein, Plasma, VDG, ndavis

Reviewed By: VDG, ndavis

Subscribers: ndavis, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D23387