Issue #77 - added UtilityMenu plugin to allow running user commands

- Added a UtilityMenu plugin that can be configured to run up to six
  different shell commands or other squirrel scripts.  Included a script
  (any_command.nut) that when run will prompt the user to enter the command
  to run and will display the output in the frontend.
- Added the fe.list_dialog() and fe.edit_dialog() script functions which
  allow scripts to prompt the user for input.
- Handle newline characters in text objects appropriately