SCREEN_PROPERTY_IDLE_MODE tied to getenv("SCREEN_IDLE_NORMAL")

Hello,

This patch lets SDL programs set the value of SCREEN_PROPERTY_IDLE_MODE via the environment variable SCREEN_IDLE_NORMAL. 

If getenv("SCREEN_IDLE_NORMAL") != NULL, then the property is set to SCREEN_IDLE_MODE_NORMAL. Otherwise, the default value is SCREEN_IDLE_MODE_KEEP_AWAKE.