provide an editor palette category listing all connectors available

Currently we only provide a single generic so called "Endpoint" item in the palette which can be configured to act as any supported connector by putting in an appropriate URI value.

In future we should replace the "Endpoints" category with a "Connectors" category providing one or more items for each supported camel connector component.

That way it is easier for new users to find the right connector and it also better advertises what is possible.