Move icon from add-on bar to the main toolbar

Review: http://codereview.adblockplus.org/5741004535627776/
Work around https://bugzilla.mozilla.org/show_bug.cgi?id=939862 preventing toolbar icon from being inserted

Review: http://codereview.adblockplus.org/5741004535627776/
Update toolbar icon visibility checks to account for the possibility that it isn't a direct child of the toolbar (Australis theme)

Review: http://codereview.adblockplus.org/5741004535627776/
Update toolbar customization to be compatible with the Firefox Australis theme

Review: http://codereview.adblockplus.org/5741004535627776/