If the theme supports, put a blur behind plasmoids

Try to make plasmoids look a bit more conform to the dialogs and panels look supporting blur behind as well. In order to support it it needs to use an hardware render api, the theme must explicitly support it via new elements (no visual change in old themes) and is disabled as well with rotated applets (perhaps is possible with a more complex custom shader, though not with the standard graphicaleffect import) all in all is all quite limited, but has an overall decent look