KCM KWinScreenEdges disable widgets if is immutable

Summary:
Same as D28507

Disable screen edge modification, but allow the user to open an edge context menu and see the settings.

To set an edge immutable, just add `[$i]` right after the entry in `ElectricBorders` group, although the edges settings are shared between some `[Effect-something]` group under the key `BorderActivateSomething`.

Since one entry like `BorderActivateSomething` lists all edges that use this effect, it doesn't make sense to set it immutable.

Test Plan:
In `kwinrc`, set the `ElectricBorders` and/or `Windows` group immutable or any entry.

```
[ElectricBorders]
BottomLeft[$i]=None
Left=None
Right=None
Top[$i]=None

[Windows][$i]
ElectricBorderCornerRatio=0.29
ElectricBorderDelay=300
ElectricBorderMaximize=false
ElectricBorderTiling=true
ElectricBorders=2
```

Reviewers: ervin, bport, meven, zzag, #kwin, mart

Reviewed By: ervin, zzag, #kwin, mart

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D28508