Remove unused passed variable from js updateDesc() function declaration
Filter saving of meta data and some refactoring

* Register meta values with their sanitation callback
* Register filters to stop add/update of meta value for default values and created the accompanying methods
* Deprecated wpseo_set_value() and changed all related function calls
* Re-factored the `get_{subset}_meta_boxes()` methods. Now one method called `get_meta_field_defs()`
* Renamed 'std' field def array key to 'default_value' to be more auto-documenting

Other:
* Code style: use phpDoc @todo tag for todo's
* Code style: else always on new line
* Code style: `else if` _not_ `elseif`
* Code style: fixed some more single line statements to have {}
* Fix few typos