Leverage Drupal Form API for CSS class plugin

Problem/Motivation

The CSS class plugin currently only supports textfield and select as input types. Especially selection of multiple classes is not very user friendly with the select element. Radios are a great input type for short option lists.

Proposed resolution

Allow to customize the UI by leverage existing Drupal form API render elements:

1. Create a new key input_type which corresponds to a form API render element #type. Additionally support the types radios and checkboxes
2. Provide sensible defaults to prevent BC breaks with existing configs: If key option is set, default the input type to select, if no options are set default to textfield.
3. Support mandatory options using by mapping the new key required to form API's #required key.