tilda_terminal: Fix Close default shell on exit

 * If the action when the custom command exits
   is "Drop to the default shell", then it was
   not possible to close the current tab by
   exiting from the default shell. Instead a new
   shell would always be spawned.
 * This commit introduces a new variable to
   remember if we have already dropped to the
   default shell. When the default shell is exited
   the second time, then we close the tab instead
   of spawning a new default shell.
 * Fixes #4