Set a timeout for prestarted mode (no shutdown before timeout finishes)

* src/modest-runtime.[ch]: added methods to access a boolean "allow_shutdown".
  Modest shutdown that happens on having empty window list and mail op queue
  will not happen if this is False.
* src/modest-main.c: only shutdown modest if allow_shutdown is false. Also
  set a timeout of 30 minutes (and allow CLI parameters to change this value).