Embed a @meta_type structure in NMMetaSettingInfoEditor and
NMMetaPropertyInfo. This allows to make the NMMeta*Info instances
themself to become generic and they can be passed around as generic
NMMetaAbstractInfo types.
For one, the embedded NMMetaType pointer can be used to determine
of which type a NMMetaAbstractInfo instance is. On the other hand,
the NMMetaType struct can be extended to be a VTable to provide
generic access to the type.
In the end, both NMMetaSettingInfoEditor and NMMetaPropertyInfo are
conceptionally very similar: the describe a certain type and provide
accessors.
In nmcli we have yet another NMMetaAbstractInfo type: NmcOutputField
will be modified to become another implementation of meta data (it
already is, it just cannot be used interchangable with the other
types).
Also, embed the NMMetaSettingInfoEditor in the NMMetaPropertyInfo
instance. This allows from a given NMMetaPropertyInfo to retrieve it's
parent NMMetaSettingInfoEditor.
"nm-meta-setting-desc.h" contains static type description, vtable and (internal)
accessor functions. Add accessor functions that operate on top of the type description
to "nm-meta-setting-access.h".
