Add a background slideshow to Cinnamon

Currently only works from gsettings/dconf. Next step is to integrate the new settings in cs_backgrounds.

Update: Now includes a modified cs_backgrounds to support the new slideshow settings.

Requires: https://github.com/linuxmint/cinnamon-desktop/pull/19

Updates:
    1) it actually works now :)
    2) re-worked treeview to be only a single level with a separator between system/user folders
    3) set the icon-view insensitive when the slideshow is activated. Mint-X needs some additional themeing to support it.
    4) properly handle multiple cinnamon-background-properties files
    5) added ~/Pictures as a default image source