Filter system revamp

Refactor the core filter.module to provide a more consistent internal API for formats and filters management, remove duplicated code and clean up current code.

Currently there are several calls to hook_filter and to hook_filter_tips withing the same function, this is because of the way that filters are defined and provided by modules through two separated hooks, hook_filter and hook_filter_tips. Multiple invokes to the hook_filter is required to obtain different properties of the filters.

Code in nested switch for $op and $delta results in hard to maintain and debug. Would be good to have filters definitions similar to menu definitions, defined in an associative array of filter properties.

Proposition

hook_input_filter
Introduce a new hook_input_filter to allow modules to provide input filters. The inputs filters would be declared using an associative array (similar to hook_menu).

Remove the concept of numeric deltas and use unique filters' names to define and identify filters.

The hook_input_filter may looks like this:

/**
 * Implementation of hook_input_filters(). Contains a basic set of essential filters.
 * - HTML filter:
 *     Validates user-supplied HTML, transforming it as necessary.
 * - Line break converter:
 *     Converts newlines into paragraph and break tags.
 * - URL and e-mail address filter:
 *     Converts newlines into paragraph and break tags.
 */
function filter_input_filters() {
	$filters['filter_html'] = array(
	  'name' => t('Limit allowed HTML tags'), 
    'description' => t('Allows you to restrict the HTML tags the user can use. It will also remove harmful content such as JavaScript events, JavaScript URLs and CSS styles from those tags that are not removed.'),
	  'process callback' => '_filter_html',
	  'settings callback' => '_filter_html_settings', 
	  'tips callback'  => 'filter_html_tips'
	);
  $filters['filter_autop'] = array(
    'name' => t('Convert line breaks'), 
    'description' => t('Converts line breaks into HTML (i.e. <br> and <p>) tags.'),
    'process callback' => '_filter_autop',
    'tips callback' => 'filter_autop_tips'
  );
  $filters['filter_url'] = array(
    'name' => t('Convert URLs into links'),
    'description' => t('Turns web and e-mail addresses into clickable links.'),
    'process callback' => '_filter_url',
    'settings callback' => '_filter_url_settings',
    'tips callback' => 'filter_url_tips'
  );  
  $filters['filter_htmlcorrector'] = array(
    'name' =>  t('Correct broken HTML'), 
    'description' => t('Corrects faulty and chopped off HTML in postings.'),
    'process callback' => '_filter_htmlcorrector',
    'tips callback' => 'filter_htmlcorrector_tips'  
  );   
  $filters['filter_html_escape'] = array(
    'name' => t('Escape all HTML'), 
    'description' => t('Escapes all HTML tags, so they will be visible instead of being effective.'),
    'process callback' => '_filter_html_escape',
  );  
  return $filters;
}

$filter array key: unique identifier of the filter
'name': Human readable name of the filter
'description': Filter description as provided by hook_filter('description')
'settings callback': Callback function that provides settings for the filter, provided by the current hook_filter('settings', $delta, ...)
'process callback': Callback function that processes (filter) the text to be filtered current hook_filter('process', $delta, ...)
'tips callback': Callback function that provides tips for the filter

For example, the tips for the filter_autop filter would look like this:

function filter_autop_tips($format) {
	return array(
    'short' => t('Lines and paragraphs break automatically.'),
    'long' =>t('Lines and paragraphs are automatically recognized. The <br /> line break, <p> paragraph and </p> close paragraph tags are inserted automatically. If paragraphs are not recognized simply add a couple blank lines.'), 
	);
}

With the new hook_input_filters, hook_filter_tips won't be required.

The database schema should be refactored as well to get rid of numeric deltas.

I am working on the patch to implement the required modifications. Feedback please!