wip: Sketch out hybrid QR transport support

[iinuwa]

• add GTK changes and QR code lib for UI flow
• Add handler trait and stream to pull into view model
• Add dummy hybrid QR handler for demonstrating flow

[AlfioEmanueleFresta]

LGTM so far (I tried running this)

[iinuwa]

This still needs a couple of things:

• UI updates after receiving BLE advert and after tunnel is established (needs support from libwebauthn)
• remove some Dummy implementations

There's a weird race condition or something: the UI renders correctly if I add breakpoints, but misses the initial update. I'm wondering if this is some sort of problem with mixing async runtimes. To avoid a big rebase, I'm going to go ahead and merge this in, switch over to Tokio every, and then come back to this.