Replaces "m_title" and "m_props".  More bloat removed.

Reduce bloat.

Call titleToPathElt() only once for each object.

Don't store in the std::map.

Don't put all <res/> attributes to the attributes map; parse the
"duration" attribute as soon as we receive it, and store it in an
integer attribute.  This reduces bloat.

At the same time, rename the enum types and the class attributes, and
add an "UNKNOWN" type/class.  The latter avoids the "-1" hack.

Return the return value, instead returning it in a reference
parameter.  Reduces bloat by reducing unnecessary std::string usage.

[mk: renamed source files, applied coding style, reduced bloat, using
MPD's threading library, using MPD's error reporting and logging
library and refactoring, fixed lots of bugs]