Plasma 6 proposal: Use KCMs for applet configuration

Whilst built-in pages such as "Keyboard Shortcuts" get to use full Kirigami.ScrollablePages, pages provided by an applet are just components loaded by a Loader inside such a page (https://invent.kde.org/plasma/plasma-desktop/-/blob/master/desktoppackage/contents/configuration/ConfigurationAppletPage.qml). This doesn't provide sufficient customisation and leads to hacks such as https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/821/diffs#a40ed9d22dbc85f71b99cec4cb2778aa2b2bc2e3 when ideally the applet will provide the page, and have full control over its appearance.

By using KCMs, we just need to provide a similar wrapper as System Settings, and we would benefit from sharing code for their pages and an identical visual result. It is also easier for applet developers, who would be able to reference settings pages for design and implementation. We would also get normal headers and header actions, which we rely on a lot in System Settings and would be good to see in applet configuration.

Plasma 6 is the ideal time to make such a breaking change.