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.
It was very confusing for every Tab to have their own GUI::Menubar that
got dynamically swapped in/out when switching tabs.
This change moves us to a single menubar per window, and BrowserWindow
is the owner of its own menubar.
Having so much the logic and lambdas in main() was getting unwieldy.
Moving it into a class simplifies this, and also opens up a path
towards supporting "Open in New Window" :^)
