[Fix] Make up missed ShowPicture in command array

As @t-nelson found, the ShowPicture command we introduced in v13 is missing from the command array in head of Builtins.cpp. Here I make it up.

As the wiki page: http://wiki.xbmc.org/index.php?title=List_of_built-in_functions
I've lost my login to the wiki system, so any dev can help make up there?