DOM: Demoted from class to functions

- The DOM object contains no members, and only two (effectively) const methods.
  Demoting DOM to function calls reduces coupling with Context.

src/Context.cpp

@@ -71,7 +71,6 @@ Context::Context ()
 , data_dir ("~/.task")
 , config ()
 , tdb2 ()
-, dom ()
 , determine_color_use (true)
 , use_color (true)
 , run_gc (true)