xowiki-www-procs.tcl

Various small improvements:

- Page->content_header_append/content_header_get: new methods to
include HTML content before the form. One use case ist to include
additional forms this way, since these cannot be included in the
main form of a FormPage. The additional content is displayed in the
www-view method.

- FormPage->create_form_fields_from_names: new convenience function to create
form-fields via their names. The definitions of the named form-fields are
taken from the provided form_constraints. This function is useful when
only a subset of the form-constraints definitions should be used
to create form-fields.

- improve comments

- improve argument checking on parent_id to be able to provide better
error messages

- reduce usage of regexp for performance reasons

- improve spelling