This changes (context) menus across the system to conform to titlecase
capitalization and to not underline the same character twice (for
accessing actions with Alt).
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" :^)
