In prefsdialog.ui change "directory" to "folder" and plural to singular

It was "folders" for two of them already, so this part is for consistency.
Singular, as it seem linguistically correct, e.g. it is "apple tree", not "apples tree". See also "chord description file".

A next step might be to combine soundfont- and SFZ folders into one preferences setting, considering that these files share a common useage and don't share the files' extention, this seems to make sense, esp. as we can use multiple folders here.