tag: converted tag_item.value to a char array

The value is stored in the same memory allocation as the tag_item
struct; this saves memory because we do not store the value pointer
anymore.  Also remove the getTagItemString()/removeTagItemString()
dummies.

src/tag.h

@@ -41,8 +41,8 @@ extern const char *mpdTagItemKeys[];
 
 struct tag_item {
 	enum tag_type type;
-	char *value;
-};
+	char value[1];
+} mpd_unused;
 
 struct tag {
 	int time;