Ensured exact field selection in _list/export.
Employed :checked selector on some radio inputs.

They had a stupid global id before AND a name. The one in map.html was also
named 'fields' and therefore manipulated by accident in the initialization.
Added filetype selection to #export form.

That xml-table is accepted by LibreOffice and Excel so I will not bother with
binary formats. I have a life!!!
Added xml table to _list/export's file types.

I employed some very crafty single-instanciated-lookup-objects to initialize
the response header without making too much fuzz. If no fields are given, the
list yields all there is, with optional geometry id and revision as the first
columns. A column will be rendered for each specified field, even if we ended
up with an empty column.