Include 'region' in $form_state in panels_edit_style_type_form

If you try to use hook_form_alter() to change the list of region styles presented to the user, you don't know which region the style is being applied to, so you use the region to decided what to hide/show.

It'd be great if the 'region' name was put on the $form_state, just like 'pane' is when setting a pane style.