Let Form API handle decisions voting form validation + submission

Currently Decisions implements decisions_voting_form_submit() and decisions_voting_form_validate() which contain some business logic and then call the relevant decisions sub-module's validation and submission functions. I agree that we should have a hook to define the function to initially define the decisions_voting_form, but I feel like the decisions_voting_form_submit and decisions_voting_form_validate functions should be eliminated and replaced with whatever submission and validation functions are defined by the form.

Motivation: As a developer, I may wish to develop a custom voting form, such as in #691782: Ajax submission, or customize the form in some way that involves changing validation and submit handlers.

Problem: I am expecting the voting form to behave like standard Drupal forms whose validation functions are the ones I would expect to see based on the form id (form_name_validate) and whichever ones are added in $form['#validate] and $form['#submit']. Instead, Decisions.module implements my forms as the contents of its own decisions_voting_form form and forces me to the accompanying code around submission and validation. The validation function provided assumes my form is structured in a particular way and the accompanying code includes a drupal_goto which interrupts the processing for an ajax submission and includes a "your vote has been registered" message that isn't appropriate in my case.

Also, rather than including the $node as part of the form values, we store only the nid and reload the node for validation, which seems inefficient.

Solution: Let's just let decisions_voting_form() call our form and let the Form API take it from there. The submodules already contain the code to validate and process submission of their forms. The refactoring would be minimal and mostly involve changing function names.