Use vertical_tabs and #states for admin settings

See title :)

The patch merges the different settings pages together and uses vertical tabs to group them together.

Additionally, it also replaces a custom jquery file with the new #states system (hiding/showing the default number of messages when the checkbox is checked).

I'm sure there is space for many more improvements (grouping, order, interface strings, summaries..), not sure if we should do it in this patch or not.

If you want to tackle it, feel free to directly work on the groups/order/strings yourself and upload a new patch. I can help you if you have any questions.

Otherwise, we can also postpone additional improvements and just go ahead by commiting this patch. Drupal 7 will be released tomorrow and I'd like to have at least a rc release out...