split up vertical_tabs_forms variable into a separate variable per form_id

Currently vertical tabs configuration for each form seems to be stored inside the vertical_tabs_forms variable. This has a major drawback though: you can not export the configuration of a specific form with Strongarm into a Features module. Therefore I suggest splitting up the vertical_tabs_forms variable into 1 variable per form, so the configuration of each form can be exported easily with Strongarm.