Change the popup of the Download Plugin action

Same behavior as today, but the popup should be not a browser native one, but a gwt one, with a message that says "You are about to download the plugin. Note it is not the easiest and productive way to use Nuxeo Studio. You should have a look at the use of the "update center", that enables hot reload for quick testing. Still, if you want to install the plugin manually,it should be placed in the $NUXEO_HOME/nxserver/bundles/plugins" folder, and you should restart the Nuxeo server.