Toolbar: Back button correctly appears/disappears on external toolbar

This commit makes the following modifications:

1. Rename _addBackButton() to _updateBackButton() and move the decision making
   as to whether to add or remove a back button from _setOptions() into
   _updateBackButton().

2. Call _updateBackButton() from refresh() as well. This will cause the back
   button to be updated whenever the page changes, because the "pageshow"
   handler is hooked up to refresh().

3. Modify _addHeaderButtonClasses to not recognize the back button as a left
   button.

4. Modify the preconditions for adding a button to include an alternative to
   checking the page's URL for instances where there is no page present - i.e.
   when the toolbar is external. In such cases one must add a back button if
   the active item on the history stack is not the first item.

Fixes gh-7091