[th/gen-metadata-nm-settings-libnm-core] generate XML file with meta data about settings (from libnm-core)

libnm-core has a lot of internal meta data about settings and properties. for example, it knows all properties (duh), their D-Bus type, their to/from-D-Bus implementation, etc.

this information can be useful outside of libnm-core. In particular, for the generated documentation. For example, man nm-settings-dbus really should list the D-Bus types (like "as", instead of "array of string" phrases).

For that, generate a new XML file src/libnm-core-impl/gen-metadata-nm-settings-libnm-core.xml. that contains this information.

So far, it is unused. The point is to use it later.