[Folder view]Make shadow scale aware

Summary: Added devicePixelRatio to shadow's radius.

Test Plan:
Check the shadow of items on the desktop.
Compare scale 100% vs big display scale (e.g. 300%).
Before: Shadow is invisible.
After: Visible shadow at every display scale.

Before vs After
Scale 300% and 100%

Reviewers: Plasma, VDG, apol, ngraham, davidedmundson

Reviewed By: Plasma, VDG, apol, ngraham, davidedmundson

Subscribers: broulik, davidedmundson, apol, ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27316