Improve support of Sidebars button in Firefox 30+

[Infocatcher]

We have:

• Cloned menu (ugh!)
• No simple way to create our menu items before showing of this cloned menu because original menu doesn't receives popupshowing event (ugh!)
• We have ViewShowing event (event.target.id == "PanelUI-sidebar"), but items is already cloned on this event.
• Also this is #customizationui-widget-panel, but this panel doesn't exist on window startup.
• We can't use window.addEventListener("popupshowing", ..., true); to wait for #customizationui-widget-panel: this doesn't work for first time.

[Infocatcher]

resource:///modules/CustomizableWidgets.jsm

  {
    id: "sidebar-button",
    type: "view",
    viewId: "PanelUI-sidebar",
    onViewShowing: function(aEvent) {
      ...
      // First clear any existing menuitems then populate. Social sidebar
      // options may not have been added yet, so we do that here. Add it to the
      // standard menu first, then copy all sidebar options to the panel.
      win.SocialSidebar.clearProviderMenus();
      let providerMenuSeps = menu.getElementsByClassName("social-provider-menu");
      if (providerMenuSeps.length > 0)
        win.SocialSidebar.populateProviderMenu(providerMenuSeps[0]);

      fillSubviewFromMenuItems([...menu.children], doc.getElementById("PanelUI-sidebarItems"));
    }

But there is no usable notifications. :(