Add two more methods to the whitelist

At application shutdown, it's very likely to be spammed by
gui-thread-check with a river of:

*** GTK CALL NOT IN GUI THREAD: Widget.Dispose
*** GTK CALL NOT IN GUI THREAD: Widget.remove_InternalDestroyed
*** GTK CALL NOT IN GUI THREAD: Widget.Dispose
*** GTK CALL NOT IN GUI THREAD: Widget.remove_InternalDestroyed
...

From what I gather, these two methods (in gtk-sharp master[1]) could
be called by the finalizers, but do not call any unmanaged functions,
so it should be ok to not report them as violations.

[1] https://github.com/mono/gtk-sharp/blob/master/gtk/Widget.cs