Horizontally center shadows and make them bigger

Summary:
FEATURE: 388256

VDG has decided that shadows should be horizontally centered and bigger. This patch implements those changes in the following way:

• Window and menu shadows are now horizontally centered
• Shadow size increased to the maximum value
• Shadow color changed from pure black to a slightly lighter Breeze standard color: Shade Black

Test Plan:
Tested in KDE Neon. Before:

After:

Reviewers: abetts, hpereiradacosta, VDG, Breeze, alake

Reviewed By: abetts, hpereiradacosta, VDG

Subscribers: rkflx, zzag, cfeck, januz, rpelorosso, apol, mvourlakos, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D9549