Support command-line configuration properties

  * Is is now possible to override any Config property using the command-line
    switch `-Dfenixframework.<configPropertyName>=value`.
  * Currently, should the programmer wish to provide all configuration
    parameters through the command-line, the minimum requirement is an empty
    `fenix-framework.properties` file to trigger configuration-by-convention.
Improve error message when no domain model is given in the configuration
Define a hierarchy for the configuration properties

  * First load `fenix-framework.properties`
  * Then load `fenix-framework-NNN.properties` (where NNN
   is the name of the BackEnd that generated the domain-specific code)
  * Finally load system properties that start with `fenixframeork.` (and
    discard the prefix)