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/WPrefs.h

@@ -111,6 +111,12 @@ void SetBoolForKey(Bool value, const char *defaultName);
 void SetSpeedForKey(int speed, const char *defaultName);
 
 
+/* ---[ double.c ] ------------------------------------------------------- */
+typedef struct W_DoubleTest DoubleTest;
+
+DoubleTest *CreateDoubleTest(WMWidget *parent, char *text);
+
+
 /* ---[ main.c ] --------------------------------------------------------- */
 void AddDeadChildHandler(pid_t pid, void (*handler)(void*), void *data);