Fix Tool configuration file dialog when selecting XRNS2XMOD Shell exe

[nrichards]

• Use file extension instead of full filename to configure dialog
• Seen on MacOS 10.13.6 / Renoise 3.1.1 64b
• Affirmed by Renoise API documentation http://files.renoise.com/xrnx/documentation/Renoise.Application.API.lua.html

Opens a modal dialog to query a filename and path to read from a file. The given extension(s) should be something like {"wav", "aiff" or "*" (any file) }

renoise.app():prompt_for_filename_to_read({file_extensions}, dialog_title)
  -> [filename or empty string]

• Make more explicit the configuration file dialog's textual
  instructions

[fstarred]

@nrichards thanks for the update, it's ok for updating the dialog title, however I have still some doubts about generalizing the exe name to a .exe because a new tool's user may have have some mistake on which exe to locate.
On the other hand the last xrns2xmod version the exe is just only one (the UI is discontinued and won't be released anymore) but is it really needed ?