Components should be stacked instead of overwritten

Here is an example of what's happening right now:

• register the component A with role R and hint H
• register the component B with role R and hint H
• unregister the component B
• there is no component anymore with role R and hint H, A is lost even if it has never been officially unregistered

Is a larger use was what this mean is the you can in XWiki install an extension which overwrite some standard component like the XHTML renderer to extend it and when you uninstall the extension you ends up with no XHTML renderer which makes XWiki pretty much unusable...

We could make each role/hint couple have a stack of descriptors assigned to it instead of a single descriptor so that we can restore the previous component when unregistering the current one.