Adjust blur offset behind plasmoid to match plasmoid itself (second try)

Previous fix somehow did not account for (literally) edge cases, such
that the background became transparent near right and bottom edges of
containment.

This patch rewrites and centralizes sizing calculations, optimizes
attached properties access, removes redundant IDs, replaces
OpacityMask::maskSource ShaderEffectSource with a simple clipped Item,
and avoid being smart about reparenting to plasmoid.

See also: 0bbba7a7

(cherry picked from commit cede2681)