add an option to copy qr code data

[r10s]

copying/pasting qr codes around is a simple and useful fallback if qr code scanning is not possible. adding this to ios is just feature parity with android/desktop in this area.

to copy a qr code to clipboard:

• in the upper right corner of the global "QR Invite Code" and also of the per-group-invite codes, add a "(...)" icon (same icon as eg. in webxdc or welcome screen)
• tapping the icon, open an action sheet with the options "Copy To Clipboard" and "Cancel"
• content to copy comes from dc_get_securejoin_qr()

to paste a qr code from clipboard:

• in the upper right corner of the global "Scan QR Code", add a "(...)" icon
• tapping the icon, open an action sheet with the options "Paste From Clipboard" and "Cancel"
• the pasted code just goes through the existing check/process pipeline as if a qr code was just scanned.

EDIT: paste becomes less important, see discussion below

[r10s]

i think, even by the new http://i.delta.chat , it still makes sense to target at least "copy to clipboard", to have the same experience on all platforms, cmp. https://github.com/deltachat/invite/issues/10 - switching later to a direct "copy invite-link to clipboard" seems to be easy then.

"paste from clipboard", oth seems not to be super-important with the new invite links; still makes sense for some corner-cases, sure, however, nothing to keep as an open issue (it was not really missed up to now).