Add pre-filling the name text presenter in file dialogs

[hernanmd]

This PR enables to preset the "name text" presenter text, which is useful for example to suggest a new file name in Save File dialogs. It also preserves the feature of updating the text when a directory or file is changed in the file navigation table.

Add tests.

Usage example:

StSaveFilePresenter new
		title: 'Save Microdown document';
		nameText: 'mydocument.' , anExtension asString;
		extensions: { anExtension }; 
		openModal.

[hernanmd]

Failing test unixLibraryName is not related.