priority for route referrers?

[dbu]

as seen when discussing symfony-cmf/routing-auto#31 and canonical route for seo, we noticed that we may need to specify that a route is more or less important for a content. in the case of redirects to the content, they could be considered less important, in case of the canonical route, it could be boosted to be more important.

one solution would be an additional interface that routes can implement to give a priority, and the generator looks at that interface (and assumes prio 0 when interface not present). i assume that the locale would remain first criteria, and priority would be second criteria. we should probably not offload this task to the content (as there may be many diferent content classes and they should not need to know about this), but do it in the generator. /cc @ElectricMaxxx, @UFTimmy