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