Only engage a GTK idle function when absolutely necessary, otherwise

the whole app spins on it and takes up CPU all the time.

[originally from svn r2052]

putty.h

@@ -484,6 +484,7 @@ void term_update(void);
 void term_invalidate(void);
 void term_blink(int set_cursor);
 void term_do_paste(void);
+int term_paste_pending(void);
 void term_paste(void);
 void term_nopaste(void);
 int term_ldisc(int option);