Accepte multiple enumerate attributes (filters)

[ger-benjamin]

Fix: #3365
Now we query each attributes to get their "description" and so, we can filter on multiple enumerated attributes (tested locally).

Note that for that, we need:

• (mapfile) That the column is given by the sql request and "exported" via the gml_include_items.
• (vars file) That each attribute is listed in the layers->enum->->attributes section
• (admin) That the layer has a enumeratedAttributes metadata with the value "column1,column2,etc"
• (Column can't have alias ! )