Tweak PhotoButton.qml

Remove camera placeholder when image is set
Keep button text even when image is set
Refactor: add SimpleButton.qml, use this for ViewFinder buttons
Issue #937: Collect images in temporary directory rather than OS
pictures folder

The images should be kept in a temporary directory which is removed when
we're done transmitting them. There's no point in keeping them in the OS
pictures folder.
Add individual image validity to identity_verification_response

The verifier can now respond that the verification was rejected due to
certain images being unacceptable.