Online resources window : add multiple selection grid with autoloaded layers if protocol is WMS getmap

**Improve link resources window with automatic load of WMS resources.**

  * link online resource :

If the protocol is one of the WMS get-map, then a multiple selection grid is displayed to choose the layers you want to link to the metadata, from the specified url

![gn-cap-link1](https://f.cloud.github.com/assets/1491924/937439/5f1f0602-00da-11e3-84ad-743181fb8a7f.png)

  * link service

When you choose a service WMS from the list, then the same multiple selection grid is displayed and you can add one or multiple layers to the metadata.

![gn-cap-link2](https://f.cloud.github.com/assets/1491924/937443/87dbc580-00da-11e3-99a0-2962aea2cc95.png)

 * link a dataset

Same as for link a service.

 * this PR also fix a bug on online resource removal. You can now remove a specific online resource depending on its url and name. Before, only the url was sent so all the online resources with the same url were deleted when you wanted to delete just one.