Fix logic for selecting action in file dialog

[greschd]

The FileDialog.action can have values open, open files, or save as, see https://github.com/enthought/pyface/blob/a9bf0264cb2afe9d05ce8856de650fd6efbc2684/pyface/ui/wx/file_dialog.py#L37 and https://github.com/enthought/pyface/blob/a9bf0264cb2afe9d05ce8856de650fd6efbc2684/pyface/ui/qt/file_dialog.py#L36.

The dialog_style on the other hand can have values open and save, see

traitsui/traitsui/editors/file_editor.py

Lines 77 to 78 in c08f891

	#: Should be one of 'open' or 'save'
	dialog_style = Str("open")

As such, the logic for converting from dialog_style to action was the wrong way around. As a result,
the file dialog would always open in "open" mode.

[corranwebster]

Looks good to me!

Thanks for the PR.

[greschd]

While running this change, I've noticed that the FileEditor doesn't properly clear its error state. I've created a follow-up PR (#2007) for this.

[corranwebster]

Could you update this to the current main branch - we've moved traitsui/qt4 to traitsui/qt and github can't automatically merge