Implement UI functions and export entry points

[neithanmo]

• remove testing code from actions.h
• implement DisplayableItem trait to certificate and inner message structures
• display ConsentMessage, this needs to be tested regarding inner message structure on a real device
• display error message description, although this could be simplified by showing only error variant description, because the received message desc could contain unsupported symbols, which need to be tested
• implement UI entry points