Allow forcing ssd drop shadows

Vlad Zahorodniirequested to merge
work/zzag/csd-ssd-shadow into master

It can be used to make some applications, e.g. Xwayland apps, look better in Plasma.

This is how Steam and Discord will look with the proposed changes:

Screenshot_20260428_203611

Screenshot_20260428_203723

Depends on work/zzag/ssd-csd-shadow branches in kdecoration and breeze.

Draft: The current KDecoration API is rather interim.

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