qml: add signals for QEQRScanner and fallbacks, add QEBytes container type
so we can pass along byte arrays between QML and python, port qr scan
occurrences to new signals.
instead, schedule a queued finished signal to unregister the listener after the handler has finished.
See PythonActivity.java in P4A for why this probably causes the most often occurring crash we see on the Play Store:
```
Exception java.lang.RuntimeException:
at android.app.ActivityThread.deliverResults (ActivityThread.java:5164)
at android.app.ActivityThread.handleSendResult (ActivityThread.java:5205)
at android.app.servertransaction.ActivityResultItem.execute (ActivityResultItem.java:51)
at android.app.servertransaction.TransactionExecutor.executeCallbacks (TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute (TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage (ActivityThread.java:2136)
at android.os.Handler.dispatchMessage (Handler.java:106)
at android.os.Looper.loop (Looper.java:236)
at android.app.ActivityThread.main (ActivityThread.java:8061)
at java.lang.reflect.Method.invoke
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run (RuntimeInit.java:656)
at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:967)
Caused by java.util.ConcurrentModificationException:
at java.util.ArrayList$Itr.next (ArrayList.java:860)
at org.kivy.android.PythonActivity.onActivityResult (PythonActivity.java:218)
at android.app.Activity.dispatchActivityResult (Activity.java:8501)
at android.app.ActivityThread.deliverResults (ActivityThread.java:5157)
```
