In gtkimcontextime.c, use gdk_win32_window_get_impl_hwnd() to get to
the impl's existing native window instead of GDK_WINDOW_HWND() which
implicitly ensures a native window for the widget itself. This seems
to work around whatever GDK problem with native subwindows and fixes
the bug.

This is based on Michael Natterer's fix for gtk-2-24.