Support ROS 2 Iron and Jazzy

The current version of P4R only supports ROS 2 distributions until ROS 2 humble in the sense that newer ROS 2 distributions cannot be selected in the configuration dialog. In the case of Iron or Jazzy, the humble settings must be used (with the possibility of an incompatibility). The current code contains String constants as well as an enumeration and an array initialization in the preference dialog. This implies that an unnecessary amount of boilerplate code needs to be changed in addition to distribution-specific code generation.