Fix a crash and extract unrelated changes

Fix a crash and extract unrelated changes

The secondary UrlNavigator is now created when and only when:
- split view mode is activated for the active tab
OR
- switching to a tab that has split view already enabled.
This fixes a crash that occurs when the setting to always start in
split view mode is enabled.

An animation for activating split view is also removed from this and
moved into a separate MR. Another unrelated name change left over from
a previous commit (viewContainers() -> activeViewContainers()) is
dropped.