o Initial implementation of variable container.
o Improve LastChange generation: we now collect all variables that
  have been changed while in the service lock and send them all
  at once. Do this automatically without explicit trigger.
o Simplify upnp_control by using the new variable change mechanism.
o print control variables as well
o clean up todo list in variable-container.
o quantize time @100ms
o Do active eventing of the current track time.
o update documentation.
o delete variable content in VariableContainer_delete
  (But, right now, we don't really care anyway - this is never called)
o documentation update.
o Don't recursively lock on time query.
o on STOPPED -> PLAYING transition, reset time.
o Only send relative time updates, when the full second changes
  (we're sampling every 500ms, so typically this is every other sample).
o update comment.
o Make logging configurable.
o Add terminal codes for more readability.