LibDevTools: Reload inspector roots after navigation

Firefox keeps using the same walker after navigation and expects it to
publish the new document root.

Normal tab debugging also relies on target switching. Re-inspect the tab
when navigation finishes, replace the walker DOM data, and notify the
watcher that the previous top-level frame target was destroyed before a
fresh target becomes available.

Send target availability as a watcher event, not as the watchTargets
response, and give each navigation a new inner window id so Firefox can
associate the new target with the new document.