Make OpenFileNameArg check that file exists

[Tom Goddard]

Would be convenient if OpenFileNameArg checked that the file exists so all the code that takes file paths don't have to do their own checks.

Eric says that the open command is currently using OpenFileNameArg to accept database ids so that would have to be changed to use a new FileOrIdArg.

[pett]

The various open/save arg types now have a 'check_existence' class variable (True/False respectively) that controls whether the file/folder name is checked for existence. The 'open' command turns off the check and does it itself as needed.