Update breeze theme shadows

Summary: I have received negative feedback (and I agree with it) on the current state of breeze shadows: they are quite dark, narrow, and feel unnatural. I tried to adress that by making shadows more sparse and a bit lighter especially on the angles while trying to keep it distinguishable when on white background. I received some positive feedback on these shadows from T10891 and the VDG channel. More specifically, I changed dialogs/background.svg to have a) longer linear gradients and b) radial gradients instead of linear on the four sides to make the center darker than the angles.

Test Plan:

Reviewers: VDG, ngraham, The-Feren-OS-Dev, ndavis

Reviewed By: VDG, ngraham, The-Feren-OS-Dev, ndavis

Subscribers: The-Feren-OS-Dev, mart, broulik, cblack, ndavis, manueljlin, ngraham, filipf, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25015