[plasmoidheading] Always use Header color by default
requested to merge ngraham/plasma-framework:always-use-header-color-for-plasmoidheader-even-when-used-as-a-footer into master
In b4dd1a18, the intended color for PlasmoidHeaders used as footers was restored.
However in my opinoin this color does not look as good as the old acidentally-always-applied Header color. So let's be explicit about always using Header colors even for footers, rather than using it by accident due to a bug.
cc @teams/vdg
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