WPrefs: Moved content of 'double.h' into 'WPrefs.h'

It is not a good idea to multiply the number of header files,
specially in this case where 'double.h' defined so few things
(and lacked the usual copyright notice / include guards).

WPrefs.app/double.c

@@ -5,8 +5,8 @@
  */
 
 #include <WINGs/WINGsP.h>
+#include "WPrefs.h"
 
-#include "double.h"
 
 typedef struct W_DoubleTest {
 	W_Class widgetClass;