csputils: provide string descriptions of chroma locations
m_property: add a sub-property mechanism

This adds a mechanism for easier export of sub-properties. The following
commits will make use of it to export fine grained information about
certain things. The sub-property mechanism reduces the amount of code
needed to export a data value to 1 line.
command: export more video params as properties

This uses the previously added sub-property mechanism to export a bunch
of stuff. For example, "video-params/w" now contains the video width.
demux: expose stream_type_name() function
m_property: add a mechanism to organize a list of sub-properties

This automatically adds a "count" sub-property, and for each entry in
the range [0, count), a numbered sub-property to access the item.
command: expose track list as properties
command: export playlist as properties
command: export chapter list as properties