Include geometry filter for download URLs, too

Fix for #1304 
In my mind, it should be like this...

There is a `collection` and there is a `filterModel` - where `collection` would be a tailored representation of what comes from geonetwork (e.g. WMS and WFS layers), and the later represents the filters (spatial, temporal yada yada).  


If we keep these things separate, then various components, given the `collection` and `filterModel`, can do with them as they like:

* the `map` can generate images to be displayed on the screen, and ignore any filter constraints as it needs to, e.g. spatial
* the subset panel can display and update the `filterModel`
* the download panel can display a `filterModel` as a human-readable, HTMLised string
* `downloader` classes can generate server requests