I18n: Add new translation sl (65%).

140 translated messages, 73 untranslated messages.

Transifex (https://www.transifex.com/projects/p/xfce/).
I18n: Update translation sl (78%).

167 translated messages, 46 untranslated messages.

Transifex (https://www.transifex.com/projects/p/xfce/).
I18n: Update translation sl (84%).

180 translated messages, 33 untranslated messages.

Transifex (https://www.transifex.com/projects/p/xfce/).