Fix invalid error message when selecting directories/folders in Import dialog

Fix invalid error message when selecting directories/folders in Import dialog

On linux SndFileSource::get_soundfile_info was being called with an empty path
value when clicking on a folder in the left side part of the file browser which
resulted in an unnecessary error message.

On Windows every time a directory was selected in the Import dialog,
SndFileSource::get_soundfile_info would fail to open the directory in read only
mode and produce an error, but as we don't want to query/open soundfile info
for directories anyway just test and return if the path is a directory.