FEATURE: Don't exit Isolate Layer mode when switching layer

Isolation mode is now saved when you move between layers.
When moving to a higher layer level , all the sub-layers are visible.

Now to exit Isolate Layer you can either toggle the checkbox in
a context menu or just Alt+click on the currently selected layer.

Reviewers: #krita, dkazakov
Subscribers: dkazakov, woltherav
Tags: #krita
Differential Revision: https://phabricator.kde.org/D2213
CC:kimageshop@kde.org
BUG:325052