Implement data validation

doc/uml/api.png

doc/uml/core-classes.png

doc/uml/cutecoin.png

doc/uml/gui-classes.png

doc/uml/models-classes.png

doc/uml/network.png

doc/uml/network.pu

doc/uml/requests.png

doc/uml/requests.pu

doc/uml/tx_lifecycle.png

src/cutecoin/core/net/api/bma/access.py

src/cutecoin/core/net/node.py

src/cutecoin/core/registry/identity.py

src/cutecoin/gui/identities_tab.py