flow: allow to specify the set of applicable query parameters per place

[x1B]

Currently, query parameter values are kept on navigation since they could be relevant to the target, and individual widgets should not be concerned with collecting/keeping parameters that are not relevant for themselves.

However, sometimes this means that query parameters start accumulating, and are kept on pages where they have no meaning. It should be possible to determine a set of "relevant" query parameters for each place. If such a set is specified, all other query parameters would be stripped. This could also serve a documentation purpose.

[x1B]

To achieve this, we will probably need to modify the internal route-map format to include an optional list of query parameters. So its a good thing that we have not implemented #393 yet.