Can't load .ufo file(folders) on macOS

.ufo shows up as a folder on macOS. However, the FileMode for QFileDialog is set to QFileDialog.ExistingFile which does not allow loading folders. (`trufont/Lib/trufont/objects/application.py line 444)

Using the default (QFileDialog.AnyFile) file mode will fix this issue. However this puts the onus on the user to open the correct file format.
I would be happy to submit a pull request if needed.