Fix for bug NB#83343

Fix for bug NB#83343
* src/modest-mail-operation-queue.[ch]:
        * Add the special handling for shutdown sync mail operation.
          This operation should be requested everytime we decide that
          modest should be finished (mail operation queue is empty, and
          no windows displayed). If queue exhaust again, and the last
          operation finished was that shutdown sync, then we can close
          modest safely. This way we don't have to sync on shutdown
          process itself.
* src/modest-main.c:
        * On (on_idle_exit_modest) handler implement checking for last
          operation. If last operation was the shutdown sync then we
          close silently. If not then we have to run the operation.
* src/modest-init.c:
        * Move to the last shutdown step the gnome vfs shutdown.
* src/modest-mail-operation.[ch]:
        * Add shutdown mail operation. This simply syncs all accounts
* src/modest-tny-account-store.[ch]:
        * New async method that shutdowns all accounts in a store.
        * Now the finalize of the account store does not shutdown the
          accounts as this step should have happened before.

pmo-trunk-r4572