win32: Fix up wintab support

We now have a proper MASTER/SLAVE input device split, where
the masters are virtual core input devices and we add fake hw
slave devices for the system pointer and real slave devices for
wintab devices.

We also set the proper source_device on the events so you can
tell which device sent it and properly decode the axis info.

gdk/win32/gdkdevicemanager-win32.h

@@ -35,8 +35,12 @@ typedef struct _GdkDeviceManagerWin32Class GdkDeviceManagerWin32Class;
 struct _GdkDeviceManagerWin32
 {
   GdkDeviceManager parent_object;
+  /* Master Devices */
   GdkDevice *core_pointer;
   GdkDevice *core_keyboard;
+  /* Fake slave devices */
+  GdkDevice *system_pointer;
+  GdkDevice *system_keyboard;
   GList *wintab_devices;
 };