Applet config dialog changes

[Nicolas Fella]

Hi,

I have an open MR that changes how applet's config UI integrates with
the container UI:
https://invent.kde.org/plasma/plasma-desktop/-/merge_requests/1675

See https://invent.kde.org/plasma/plasma-desktop/-/issues/99 for some
context.

The benefits would not only be somewhat cleaner internals but also more
flexibility for the applet itself. Currently the container provides a
scrollview around the config, which is problematic for content that
wants to provide its own scrollview, like the system tray entries page.
See https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/3204

The downside is that we need to touch all applet configs. Most of this
is pretty straight-forward though, replacing the root item with a
SimpleKCM (or other appropriate *KCM type).

With release/freeze coming up we're in a now-or-never situation here.
Should we do a concerted push towards getting this done?

Cheers

Nico