"On The Go" Slideshow functionality

[jingai]

Implement feature to add and edit Slideshows from within the plugin.

Notes to myself:

• slideshow_add gets its data from photo_list()
• clean up context menu items in photo views
  • Picture information = 13406, window = 10139
    • Start slideshow here = 13422, window = ???
• document iPhoto smart albums vs slideshows
• add delete context menu item if current image or folder is in slideshow
  • only refresh view if in Slideshow (use Container.FolderPath?)
• function to scan slideshow for images that have been deleted from main database