Add FilterPage you can use to filter a WMS source

- Added FilterPage class which links to a WMS source. The WMS source's
  featurelines can be configured on the FilterPage as available filters.

- There's a view for the FilterPage that shows the available filters as
  dropdowns. The dropdown values depend on the values that can be found in the
  bounding box. Submitting the form filters on that value. TODO: make the
  interaction more dynamic (read: javascript).

- A CSV export of the filtered data is available.

- Current limitation: max 100 items on the filterpage.