[ADD] doc for data files

* remove remnants of <wizard>, <getitem>
* remove <field ref="null"/>
* don't document <url>, <assert>, <ir_set>, <act_window> because they're crap
* attempt to cleanup rng and code a bit
* ignore icons in <menuitem>

Various attributes not documented (workflow/@uid, data/@noupdate,
data/@context), also <field type="int">None</field> shall remain undocumented.