Handle enumerations with the full range of signed and unsigned values

The C compiler will pick an enumeration type that accomodates the specified
values for the enumeration, so ignoring 64-bit enumerations, we can
have enumeration values from MININT32 to MAXUINT32. To handle this properly:

 - Use gint64 for holding eumeration values when scanning
 - Add a 'unsigned_value' bit to ValueBlob so we can distinguish the
   int32 vs. uint32 cases in the typelib
 - Change the return value of g_value_info_get_value() to gint64.

https://bugzilla.gnome.org/show_bug.cgi?id=629704

gitypelib-internal.h

@@ -632,6 +632,7 @@ typedef struct {
 /**
  * ValueBlob:
  * @deprecated: Whether this value is deprecated
+ * @unsigned_value: if set, value is a 32-bit unsigned integer cast to gint32
  * @value: The numerical value
  * @name: Name of blob
  *
@@ -639,8 +640,9 @@ typedef struct {
  */
 typedef struct {
   guint32 deprecated : 1;
+  guint32 unsigned_value : 1;
   /* <private> */
-  guint32 reserved   :31;
+  guint32 reserved   :30;
   /* <public> */
   guint32 name;
   gint32 value;