kcms/tabbox: Use higher resolution thumbnails for previews

Ismael Asensiorequested to merge
work/iasensio/tabbox-preview-thumbnails into master

Replace the hardcoded thumbnails which have a low resolution for current screen standards (~300x200), with higher resolution ones (~1200x800).

This is more noticeable on switchers with larger thumbnails.

BEFORE AFTER
tabbox-thumbnails-before tabbox-thumbnails-after
tabbox-thumbnails-cover-after tabbox-thumbnails-cover-before

BUG: 446765
FIXED-IN: 5.93.0

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