We currently use LibWebView's Application as the entry point to learn
about the bookmarks bar being shown/hidden, and propagate that through
virtual methods. At the time this was added, AppKit's Tab window and
Qt's BrowserWindow did not have a settings observer. They do now, so
let's skip a couple of middle-men.
For AppKit, we change the settings observer to just (weakly) store the
Tab instance so that we don't have to add callback functions for each
setting.
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