Browser: Add "Inspect Element" to context menu :^)

This opens the DOM Inspector window, with the target element already
selected. (If the window is already open, it just selects the element.)

Note that this only applies to single-process mode for now. In OOP mode,
the "inspect element" action is disabled.

Userland/Applications/Browser/InspectorWidget.h

@@ -18,6 +18,7 @@ public:
 
     void set_document(Web::DOM::Document*);
     void set_dom_json(String);
+    void set_inspected_node(Web::DOM::Node*);
 
 private:
     InspectorWidget();