add an option to copy+paste 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.