Add page for scanning and sharing QR codes

Currently, the page can only add contacts by scanning QR codes which
contain XMPP URIs with their JIDs.

The chat page is opened when the scanned contact is already in the roster.
A passive notification is shown when the URI is invalid.

Co-authored-by: Mathis Brüchert <mbblp@protonmail.ch>
3 jobs for master in 2 minutes and 33 seconds (queued for 140 minutes and 36 seconds)
Status Name Job ID Coverage
  Test
passed build #128585

00:01:01

passed clazy #128583

00:01:10

passed qmllint #128584

00:00:20