When generating output data, nmcli iterates over a list of
property-descriptors (nmc_fields_ip4_config), creates an intermediate
array (output_data) and finally prints it.

However, previously both the meta data (nmc_fields_ip4_config) and
the intermediate format use the same type NmcOutputField. This means,
certain fields are relevant to describe a property, and other fields
are output/formatting fields.

Split this up. Now, the meta data is tracked in form of an NMMetaAbstractInfo
lists. This separates the information about properties from intermediate steps
during creation of the output.

Note that currently functions like print_ip4_config() still have all the
knowledge about how to generate the output. That is wrong, instead, the
meta data (NMMetaAbstractInfo) should describe how to create the output
and then all those functions could be replaced. This means, later we want
to add more knowledge to the NMMetaAbstractInfo, so it is important to
keep them separate from NmcOutputField.