add sign message feature to android qml

[pony-montana]

Is this feature well hide or is it missing in qml-android? Will be great to have it!

[SomberNight]

sign_message is not implemented in the qml gui atm. (It was not in kivy either)

[BeatGubser]

I would also appreciate the sign/verify message feature for android qml very much.
A sign/verify message for the lightning node in android qml would also be great.
Thank you!

[accumulator]

I had a PoC sitting in a local branch for a while, I finished it up and it is ready for review.
#8623

[pony-montana]

tested #8623 it works pretty well! Just a bit strange the floating text-boxes (where the user write messages and signs), but nothing that ruins the experience. I dont know if could become problematic for small screens or with very long messages that requires "to scroll".