Add a drupal_build_form() helper for using a method as the callback

Thanks to call_user_func_array(), forms can be built using any callable, not just functions.
However, the steps to do this are rather confusing, and it's hard to find examples of how to do it.

I think another drupal_*_form() function is a small price to pay for the utility of this, especially in the D8 world of increasing OO.

Since it is a straight API addition, it should even be backportable.