Drop shadow tweaks for delegates

Summary:
Make shadows centered and slightly larger. Fits better with other shadows in the
desktop (like dolphin and widgets), and it also aligns with the new shadows for
breeze (D9549).

Current:

Patch:

Reviewers: Elisa, ngraham, astippich

Reviewed By: Elisa, ngraham, astippich

Subscribers: mgallien, ngraham

Tags: Elisa, VDG

Differential Revision: https://phabricator.kde.org/D9681