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.

src/misc/mod.rs

@@ -1,11 +1,15 @@
 mod calculator;
+mod captcha;
 mod clock;
+mod scanner;
 mod stopwatch;
 #[cfg(all(test, feature = "test-support"))]
 mod tests;
 mod units;
 
 pub use calculator::Calculator;
+pub use captcha::{Captcha, CaptchaState};
 pub use clock::{Clock, WorldClock};
+pub use scanner::QrCodeScanner;
 pub use stopwatch::Stopwatch;
 pub use units::UnitConverter;