Inspector: use NavigationSidebarPanel.createTreeOutline constructor to make the probes tree outline. Add an empty message for the probes row.
Inspector: specialize a DetailsSectionRow for TreeOutlines. Don't automatically attach
tree outlines created by NavigationSidebarPanel.createContentTreeOutline().
Rename NavigationSidebarPanel.contentTreeOutline to defaultContentTreeOutline.
Inspector: move NavigationSidebarPanel.applyFiltersForTreeElement to TreeElement.applyFilter.
Inspector: move remaining tree-traversal code for filters into
TreeOutline. Adjust initialization order within
NavigationSidebarPanel.
Inspector: parameterize how NavigationSidebarPanel handles changes to the filter.
Then, teach DebuggerSidebarPanel how to track and filter multiple TreeOutlines.
When the tree outline doesn't match the filter, show an empty message in the outline's
detail section row. (This last part could use better messages.)

Allow subclasses of NavigationSidebarPanel to change the placeholder messages
without immediately displaying said message.
Fix an errant use of the BSD 3-clause license. We meant to insert a 2-clause license.