add multi-device-setup

[r10s]

requires deltachat/deltachat-core-rust#4007 to be checked out

counterpart of deltachat/deltachat-android#2493

replaces #1649

the layout is not perfect, but this is what we could achieve with the limited resources :)

todo:

• wrapper
• basic UI
• show progress, hide QR code once scanned (this is also nice feedback)
• consider calling triggerLocalNetworkPrivacyAlert() on receiving, EDIT: in contrast to [WIP] Iroh share UI #1649 there are currently no crashes when the alert is shown "in interrupted core", however, it still seems advantageous to have a bit more control, if things shift around
• consider calling triggerLocalNetworkPrivacyAlert() on sending - we did not do this that time in [WIP] Iroh share UI #1649 - is it needed? one should try out of the alert pops up at all (make sure, to be on an installation where the alert was not triggered before) - EDIT: yes, it makes sense for the same reasons on sending
• use "Cancel" instead of back button to make also visually clear, things are aborted when leaving
• keep transfer screens on
• add a warning when one wants to leave the transfer screen during transfer
• add dedicated "Add as Second Device" button to welcome screen
• if wording is settled on android, pull it from there
• add "experimental" hint to all places
• improve layout, at least of sender (for a MVP, the receiver is good enough)
• add a confirmation dialog before showing the qr code
• add menu with "troubleshooting" and "copy to clipboard" to sender
• fix: scanning from the in-app-qr-code scanner brings you to the welcome screen, but without action

optional for 1.36:

• sender-layout for landscape
• add progress bar instead of showing percentages (percentages may be out of sync with receiver)
• consider checking system secret before sending transfer, link, however, for 1.36, this seems optional

counterpart of deltachat/deltachat-android#2493