Add layer manager component

This PR adds a very first version of the "layer manager" component.

This is really a preview version, as a number of issues remains:

 * styling is not good
 * doesn't integrate well with "import kml" and "import wms"
 * when a layer is removed from the layer selection, it doesn't get unchecked in the catalog

I'd like to get this PR reviewed, and address the rest with other PRs.

This PR is based on #309, which should therefore be reviewed and merged first.