Send ButtonRequests from Rust UI

[matejcik]

one thing we still need Python glue for is sending ButtonRequests when going from one flow step to another. This is a problem e.g. in the show_address feature, where there are supposed to be multiple button requests when going from address -> qr -> xpub -> address etc., but it can't be implemented as a single sequence in Rust because Rust is incapable of emitting the appropriate button requests.

It should be possible to expose "send button request" callback similar to how "request timer event" works.