Use the global save button to save the media library properties (closes #34)
For this a method save() has been added to IFrame as the managed folders aren't being configured through pmsConfiguration, but directly through MediaLibraryStorage (functionality could be added to MediaLibraryConfiguration)
 - Managed folders are being saved when clicking save
 - Prefixes to omit are being saved (and used) as soon as the set text changes
 - The cover save folder path is being saved (and used) after having a folder from the file chooser dialog. (Pictures having been saved in the current folder are NOT being moved and wont be usable for videos for which a cover has already been imported)