Filename parameter should allow non-existent files

[jbednar]

If filename is used to select from existing files, it can be useful for it to show (invalid) in Panel when the file doesn't exist:

But it's also important to be able to ask the user for a filename to save to, so it seems like there should be a setting that doesn't warn on non-existent files, and indeed might warn on existing files. Presumably it should still check that the path is valid, though.

[maximlt]

Just noticed that looking at the code, this is also a valid feature request for Path and Foldername.