allow scanning an `DCACCOUNT:` qr-code from the system camera and by the internal scanner

[r10s]

• currently, if you scan a DCACCOUNT: qr-code using the system camera, nothing happens. it would be nice if instead Delta Chat is opened and the user is asked "Create new e-mail address on 'provider.org' and log in there? bla bla ..." (we can use qraccount_ask_create_and_login_another as it is used on android already).

• similar when scanning a DCACCOUNT: qr-code from within Delta Chat's qr-code-button in the lower left corner of the chatlist - there it says "The scanned QR code is for setting up a new account [...]" - it could also show the same question and forward to the welcome screen.

(nb: scanning a qr code from the camera app and opening delta chat already works for secure-join and join-group qr-codes(OPENPGP4FRP:), so we probably can copy that pattern :)