Resolve WB-661 Open dialog file filter not functioning on Mac: Changed the...

Resolve WB-661 Open dialog file filter not functioning on Mac: Changed the CaretFileDialog so that it uses Qt's file dialog (instead of a native dialog) since the proxy file filter only works with a Qt file dialog.  The proxy file filter is needed to hide files that do not match the selected file filter and to prevent CIFTI files from being listed when the file filter is set to volume files.