Various improvements to QR scanner

* Improve QR scanner initialization time
* Fix race condition with QR scanner and button spamming
* Don't show black screen before QR scanner has loaded
* Show loading message for QR scanner

include/camera.h

@@ -37,6 +37,7 @@ typedef struct {
     volatile bool finished;
     volatile bool success;
     Handle cancel;
+    Handle started;
 
     bool capturing;
     struct quirc* context;