Implement frameless style for configuration pages with ListView

Frameless style is something we've been doing a lot throughout Plasma lately, because it looks modern, polished and streamlined.

Unfortunately, it requires a bit of hack to temporarily override/reset padding of the configuration containment. But the hack is harmless: if upstream changes layouts so that the hack fails to find the Page, then it just won't do anything, i.e. fails silently.

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