Add an enum type for specifying that we don't know the type (i.e. the key

* preferences.c: (bg_preferences_load),
	(bg_preferences_merge_entry), (read_wptype_from_string),
	(read_color_from_string), (bg_preferences_get_wptype_as_string):
	* preferences.h:

	Add an enum type for specifying that we don't know the type (i.e. the
	key was not available, or set to something invalid). Currently it
	was using -1, which barfs with some compilers that set enums
	to be uint.

libbackground/preferences.h

@@ -52,7 +52,8 @@ typedef enum _orientation_t {
 
 typedef enum _wallpaper_type_t {
 	WPTYPE_TILED = 0, WPTYPE_CENTERED, WPTYPE_SCALED,
-	WPTYPE_STRETCHED, WPTYPE_EMBOSSED, WPTYPE_NONE
+	WPTYPE_STRETCHED, WPTYPE_EMBOSSED, WPTYPE_NONE,
+	WPTYPE_UNSET
 } wallpaper_type_t;
 
 struct _BGPreferences