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:
{F5587393}

After:
{F5587390}

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