Drop most per-activity power settings

Currently there's four different modes for per-activity power settings

1) No special settings

2) "Act like" something else, where something else can be Battery, AC, or another activity. The UI
for this mode has been broken due to a bug for seven years (1cf565ee)
Apparently nobody noticed or missed it

3) Define a special behavior, like inhibiting screen power management or suspend. Usually this shouldn't
be needed, because applications where either of them is relevant should request inhibiting those themselves.
Alternatively it is possible to manually inhibit power management from the applet. I can see how this is potentially
useful though

4) Completely custom per-activity settings. This is basically the regular settings, applied to a specific activity only.
This makes the UI very complex, and appears very niche without apparent tangible use cases

This change removes 4) due to the ratio of code and UI complexity to usefulness

2) is removed too, given it has been broken long enough

Fixes plasma-desktop#60