Do not take over config.xml files on other project types

Adds the config.xml content type to nature, also enhances the content
describer to avoid name conflicts.

Signed-off-by: Gorkem Ercan <gorkem.ercan@gmail.com>
Revert to the standard eclipse describer

reverts to use the standard describer as it provides the same
functionality as ConfigFileContentDescriber

Signed-off-by: Gorkem Ercan <gorkem.ercan@gmail.com>