Use base URL for AJAX request to `set_default_tab`

Somehow the AJAX request to `@@tabbed_view/set_default_tab` ends up going to

- `container/@@tabbed_view/set_default_tab` instead of 
- `container/context/@@tabbed_view/set_default_tab` 

for non-folderish contexts.

Unlike regular HTML links, the jQuery `$.ajax()` call [ignores the base URL](http://stackoverflow.com/a/9079801/1599111) when creating requests from relative URLs. So one would expect that it behaves the same like regular links in HTML do: A relative URL `foo` will turn into

- `http://container/context/foo` for a location of `http://container/context/` (trailing slash), and into
- `http://container/foo` for a location of `http://container/context` (no trailing slash).

However, this does not seem to be the case. The request at [`tabbedview.js:521`](https://github.com/4teamwork/ftw.tabbedview/blob/988b9bd566147cfd61ed2ba90121fd02dbe840d1/ftw/tabbedview/browser/resources/tabbedview.js#L521) always ends up going to `container/@@tabbed_view/set_default_tab`, with trailing slash or without.

Therefore I change the AJAX call in this PR to always build absolute URLs using the base URL (which Plone sets correctly).

@maethu @jone please review.