Reorganize filter_api operations

Move some code around filter_api and related code, to clean up, and
remove duplicated logic.
The code is now organized in three separated main functions:

• filter_get default(), filter_get_default_array()
  This creates an initial filter array, with all properties needed,
  initialized to its default values

• filter_ensure_valid_filter()
  This validates a filter array, changes types, adds missing
  properties.
  Missing properties are filled with defaults, instead of reading
  values from post/get parameters

• filter_gpc_get()
  New function to perform the reading of parameters from get/post.
  A filter array can be passed as parameter, then the read parameters
  will be appended, or everride existing ones.
  If no filter array is used as parameter, a default one will be used.
  The gpc read of parameters has been removed from view_all_set.php,
  and now it can be used from the filter_api.