Misc · Codes and checks: a QR code scanner and a captcha

misc::QrCodeScanner reads QR codes with rqrr 0.11 from the frames the
host hands in, one at a time off the main thread, and hands each code
to the owner once. misc::Captcha shows the owner's picture over an
answer field that empties and keeps focus with each answer; the owner
says how it stood. BarcodeDisplay, Signature and Rating Widget point
to their homes.

Cargo.toml

@@ -29,6 +29,7 @@ log = "0.4"
 pulldown-cmark = { version = "0.13", default-features = false }
 qrcode = { version = "0.14", default-features = false }
 regex = "1"
+rqrr = { version = "0.11", default-features = false }
 rust-embed = "8.12"
 serde = { version = "1", features = ["derive"] }
 serde_json = { version = "1", features = ["preserve_order"] }