Babelview: use drop-down for languages instead of buttons when there are too many translations

On the babel view, when there are a lot of translations, the line of buttons for those translations can become too large to fit comfortable into the left part of the form:
![Screen Shot 2013-03-13 at 10 06 53 ](https://f.cloud.github.com/assets/642660/252970/e36814f4-8bbd-11e2-81f9-c484fa625026.png)

This pull request adds an option to use a drop-down menu instead of buttons if there are more than X translations. This number can be set in the "Extra options" of the control panel and defaults to 7. Setting this option to 0 will mean that buttons are always used.

With the threshold set to 7 the example above would then look like this:
![Screen Shot 2013-03-13 at 10 07 28 ](https://f.cloud.github.com/assets/642660/252988/7b394d48-8bbe-11e2-8a7d-9aadd7feff95.png)

If this request gets accepted, I will submit a follow-up request for archetypes.multilingual, where the template needs to be adapted (the @@babel_utils view already has the necessary method).