Added new field type "section"

With this field type you can now add additional h3-headlines between
options on the tabs pages.  It is also possible to indent the options
after a section element a bit to the right to make them look like a
group of options. This way can get a clearer structure into the tabs content when you have a lot of options on one page/tab:

![section-field2](https://f.cloud.github.com/assets/5825510/1551358/8ff095a4-4e12-11e3-99c1-fc2d45d19b29.jpg)