Support "activate" request from foreign-toplevel protocol

[k0ral]

This is somewhat a follow-up of the following comments from another issue

• https://github.com/riverwm/river/issues/350#issuecomment-1019315752
• https://github.com/riverwm/river/issues/350#issuecomment-1020621976

Notably:

But there is a catch: River currently does not allow it to focus windows that are not already visible on an output. Why is a different discussion however (feel free to open an issue about this).

I couldn't find any such issue, so that comment made me feel justified to open the present issue.

Has the situation evolved since then ? If one were willing to contribute to help things move, where would you suggest to start ?