Make module-defined date types usable

Problems

1. Date types have default formats. That's a fact. We want format_date($timestamp, 'mytype') to work even if the administrator has not configured a date format for that date type. For that to happen each module that provides a date type currently needs to implement hook_install():
   

   function mymodule_install() {
     variable_set('date_format_mytype', 'g:i:s \o\r \w\h\a\t\e\v\e\r');
   }
   

   That's very cumbersome and unintuitive (and senseless).

2. Because of the nature of format_date() a date type with the machine name 'custom' doesn't work. That's because if you specify 'custom' as the $type parameter that allows to specify a custom format in the $format parameter. This is a completely unneccessary limitation.

Solutions

1. This patch changes the return value of hook_date_format_types():
   

   // Before.
   mymodule_date_format_types() {
     return array(
       'mytype' => t('My type'),
     );
   }
   
   // After.
   mymodule_date_format_types() {
     return array(
       'mytype' => array(
         'title' => t('My type'),
         'default format' => 'g:i:s \o\r \w\h\a\t\e\v\e\r',
       ),
     );
   }
   

   We use 'title' instead of 'label' here because the title/label is already stored as 'title' in the DB. _system_date_format_types_build() now automatically takes care of setting the variables that had to be set manually before. Ideally we wouldn't set a variable at all, but simply invoke hook_date_format_types() in format_date() and access the default format from there, but format_date() can be called before full bootstrap, so that isn't possible.

2. This patch splits format_date() into format_date(), which is responsible for formatting date types, and format_date_format(), which is repsonsible for formatting date formats. Examples:
   

   // Before.
   format_date($timestamp);
   // After.
   format_date($timestamp);
   
   // Before.
   format_date($timestamp, 'mytype');
   // After.
   format_date($timestamp, 'mytype');
   
   // Before.
   format_date($timestamp, 'custom', 'g:i:s \o\r \w\h\a\t\e\v\e\r');
   // After
   format_date_format($timestamp, 'g:i:s \o\r \w\h\a\t\e\v\e\r');
   

   The $timezone and $langcode parameters are retained as before and available in both functions. Internally format_date() just gets the format from the variable (that was saved earlier) and then hands it over to format_date_format.