consider using WFS DescribeFeatureType to get list of attributes

[bartvde]

this is a follow-up from #77

in some cases it might make sense to have a drop down of all the available attributes on vector layers, it's hard for the user to know their dataset well otherwise

[bartvde]

but there doesn't seem to be a clear way for the client to determine that something expects an attribute as its value, except for the name 'attribute'

            <Input maxOccurs="1" minOccurs="0">
                <ows:Identifier>aggregationAttribute</ows:Identifier>
                <ows:Title>aggregationAttribute</ows:Title>
                <ows:Abstract>Attribute on which to perform aggregation</ows:Abstract>
                <LiteralData>
                    <ows:AnyValue/>
                </LiteralData>
            </Input>

so this might not really be feasible

[bartvde]

maybe we could have a button where the user can get it on demand?

[bartvde]

but we don't have a parser for this in ol3 or JSONIX, so I'm going to discard this now