Track whether _finish() has been called, and deliver all partial
results then.  Fixes Mantis ticket 0003621.

Rationale: vanilla libid3tag does not have any pkg-config stuff
and fails to detect because symbols from libz are not found.

- fix potential memory leak of system_path

  'Potential' because currently g_get_system_config_dirs()
  returns single entry on Windows, but that might change.

- remove incorrect g_free() call

  It's not required at all because
  g_get_system_config_dirs() returns GLib owned memory.

- remove extra semicolon

Prepare for adding non-trivial attributes.

Use the C++ API.