Remove theme_checkboxes() and call theme('container') instead

Problem/Motivation

There are way too many theme functions / template files in core. Some are similar (if not identical) and should be consolidated.

Proposed resolution

We have two options:

1. Remove theme_checkboxes() and call theme('container') instead (build a render array with '#theme' => 'container')
2. If we really do need a separate preprocess function for this, let's use theme('container__checkboxes') instead of theme('container') and add all our preprocess magic into template_preprocess_container__checkboxes()

Remaining tasks

Decide which of the 2 options above to pursue, and do it.

User interface changes

None

API changes

Removal of theme_checkboxes

Related Issues

#2041825: Remove theme_radios() and call theme('container') instead
#1819284: [meta] Consolidate all form element container templates, and add theme_hook_suggestions