No load progress bar on offline content

[sonnyp]

Loading from disk is instant and the progress bar makes it feel slow

[sonnyp]

I also wanted to do

    this._webview.bind_property(
      "is-online",
      this._toolbarview,
      "extend-content-to-top-edge",
      GObject.BindingFlags.SYNC_CREATE | GObject.BindingFlags.INVERT_BOOLEAN,
    );

but it hides the backdrop in the webview behind the header bar we would need to tweak the inject js/css but it's tricky so let's skip it for now