Use PowerDevil::ActivitySettings instead of KConfig

Straightforward port to type-safe KConfigXT config accessors.
Replaces direct access to powermanagementprofilesrc for its
"[Activities]" config group. (The migrated config values are now
located in powerdevilrc for Plasma 6.)

Given the reduced scope of activity settings after previous
Plasma 6 changes, there is no point anymore in keeping a
distinction between "Define a special behavior" and
"Do not use special settings". The latter is implied if neither
of the two special setting checkboxes is enabled.

Across global and activity settings, as well as profile settings
going forward, each settings object uses a KSharedConfig internally.
Saving one won't undo any changes that another settings object
may have written to the same config file.