#8 'Add folder's content to current playlist tab' button

[Stephen Ash]

When clicking this button, the contents of the selected folder should be added to the currently selected playlist. Instead, a new playlist is created.

Repo steps:
1. Open 'Media' tab
2. Select desired playlist tab
3. Click 'Add folder's content to current playlist tab' button
4. Navigate to folder and click 'OK'
5. Files are added to a new playlist

Workaround:
1. Add contents to new playlist
2. Select all entries in new playlist
3. Right click on entry and select original playlist from menu
4. Close new playlist