Web Inspector: clarify naming conventions for using cookies to
restore represented objects and view state of content views.
Web Inspector: Fix a bug where childless TreeElements never look
expanded even when children are later added and selected.
Probes: show number, string, null, bool samples in the probe data grid.
Workaround for an assertion where multiple back-forward list items
think they are visible at the same time.

https://bugs.webkit.org/show_bug.cgi?id=124685
Web Inspector: restore the last content view state the main resource changes.

Closes #412.