To fix #18964, pqMultiViewWidget no longer
automatically grabs a newly created view. Instead association of a view
with a layout is now an explicit action. This makes it possible to
faithfully reproduce Python traces when played back in the Qt GUI.

Since all views are now created 'detached', the APIs to create detached
views is being removed since it's no longer applicable.

Python state support was also imporved to correctly record view layouts.
Previously, layouts were simply not saved in Python state files.

SplitViewTrace and TraceMultiViews tests now do baseline compares to
validate the fixes.

Fixes #18964.