QCILViewerWidget add enableSliderWidget argument

[DanicaSTFC]

Describe your changes

• remove kwargs from QCilViewerWidget, QCILDockableWidget
• tested that setting the argument enableSliderWidget to False hides the slider
• unit test pass (apart from failing unit test: pytest and QApplication #367)
• unit tests not added. An issue has been opened to take care of this Add tests for QCILViewerWidget #397
• not tested the web viewer (see Web viewers errors #400)

Describe any testing you have performed

Consider adding example code to examples

Standalone app:
I pip installed and tested with numpy magma data. Moved the slider in the 2D viewer and 3D viewer and spanned z with the mouse
I then set the enableSliderWidget to False and checked it did not appear int the viewer
I ran all the unit tests.

Link relevant issues

closes #384

Checklist when you are ready to request a review

• I have performed a self-review of my code
• I have added docstrings in line with the guidance in the CIL developer guide
• I have implemented unit tests that cover any new or modified functionality
• CHANGELOG.md has been updated with any functionality change
• Request review from all relevant developers
• Change pull request label to 'waiting for review'

Contribution Notes

• The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in CILViewer (the Work) under the terms and conditions of the Apache-2.0 License
• I confirm that the contribution does not violate any intellectual property rights of third parties

Qt contributions should follow Qt naming conventions i.e. camelCase method names.

VTK contributions should follow VTK naming conventions i.e. PascalCase method names.