cubicweb-searchui #4037153 Implement a relation switch facet [validation pending]

The facet should inspect the current rset and selected variable and display a list of relations that can be traversed from the selected variable.

When the user clicks on one of those relations, the RQL is updated to follow this relation and select the variable at the other end.

It should also be possible to specify a list of rqlpaths for more complex cases.

done in0.1.0
load left0.000
closed by<not specified>
patchupdate README (related to #4037153) [applied]implement rqlpaths (related to #4037153) [applied]i18n roundup (related to #4037153) [applied]first implementation of RelationSwitchFacet (related to #4037153) [applied]