Remove borders if showing a Representation instance

if the applet uses a Representation component as full representation it means will be the contents itself managing own paddings, so shouldn't be the containers adding extra margins. do this only if the plasma theme has an explicit inset hints so that's "ready" for this

Depends from: https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/156

Related: plasma-desktop!246 (closed)