Fix labelset widget issues

Summary:
Observe relevant event for new layers

Each layer is a label set so creating a new layer
requires to observe the label set events for this new layer / label set.

Do not explicitly react on label changes

This is already done by OnEstablishLabelSetConnection:
The function allows to observe different events being sent.

Get active label on active layer

Always use a defined argument for the layer.
Otherwise, layer 0 is used as a default value.

Test Plan: - change label color on layer > 1 and see the immediate effect

Reviewers: kislinsk, O1 MITK Reviewer Group I

Reviewed By: kislinsk, O1 MITK Reviewer Group I

Subscribers: kalali, kislinsk

Maniphest Tasks: T29110

Differential Revision: https://phabricator.mitk.org/D632