Fix MSW implementation of getSaveFilePath and getOpenFilePath to respect initialPath

This PR updates the MSW implementation of these two utility functions so they behave as expected (and as they behave on OS X). That is, if you supply an initialPath that is a directory, the dialog box will open in that directory, and if you supply an initialPath that is a file, the dialog box will open in the proper directory and pre-populate the filename field. If you don't supply an initialPath the functions behave as they currently do (no change).

Fixes #68