Added code to hook low-level keyboard events on 2K/XP/Vista.

The hook is only installed if in full-screen mode.
Filters out the left and right Windows keys to avoid accidentally dropping
out of full-screen mode.
If Tux Paint is deliberately switched to the background (using ALT-TAB, say)
the Windows keys function as normal until Tux Paint is the active
application again.

src/win32_print.h

@@ -24,4 +24,9 @@ extern char *GetDefaultSaveDir(const char *suffix);
 extern char *GetSystemFontDir(void);
 extern char *get_temp_fname(const char *const name);
 
+/* keyboard hooking functions */
+extern int  InstallKeyboardHook(void);
+extern int  RemoveKeyboardHook(void);
+extern void SetActivationState(int state);
+
 #endif