Fixed bug: Loosing separate IO ports when loading.

* description of bug
    * loading a session when SL is already running would not restore
      separate (Jack) IO ports
* analysis
    * new loops (class Looper) were added before old loops were
      destroyed
    * Jack ports are created / destroyed upon Looper creation /
      destruction time
    * hence, tried to create Jack ports that already existed (because
      they were not destroyed yet); which fails
* fix
    * loading a session is now a two phase process
    * phase 1 deletes all existing loops
    * phase 2 parses xml file and creates new loops
    * phase 2 is only started when phase 1 is done (both normal and rt
      thread completed deleting all loops)
* while loading a new session, add and remove loop commands are
  disabled (just to be on the safe side)