Add a displayer for SpaceReference parameters

The idea is to implement a new org.xwiki.model.reference.SpaceReference type for macro parameters.

This would solve a pretty common use case for macros, where we pass a "location" parameter in order to scope the effect of the macros to the given subtree. (For now it is not possible to have a picker to modify such location parameter of a macro, the closest we can get is using the org.xwiki.model.reference.DocumentReference type for the macro parameter, but there are several limitations with this approach, as we then need to handle the `WebHome` page of the space instead).