Ladybird: Reimplement the DOM inspector :^)

This has been broken since the switch to the multiprocess architecture
(and even before then was very limited).

This restores the previous functionally and also implements the ability
to inspect individual elements (by selecting them in the tree view).
The inspector also now correctly updates when navigating between pages.

Ladybird/CMakeLists.txt

@@ -82,6 +82,7 @@ set(SOURCES
     ${BROWSER_SOURCE_DIR}/History.cpp
     BrowserWindow.cpp
     ConsoleWidget.cpp
+    InspectorWidget.cpp
     ModelTranslator.cpp
     Settings.cpp
     SettingsDialog.cpp