Disposable: Emit 'wasDisposed' on destruction, not dispose()

Fixes segfaults when objects that were not explicitly disposed
were collected by the GC.

binding-mri/disposable-binding.h

@@ -70,9 +70,6 @@ RB_METHOD(disposableDispose)
 	if (!c)
 		return Qnil;
 
-	/* Inform core */
-	c->wasDisposed();
-
 	disposableDisposeChildren(self);
 
 	delete c;

binding-mruby/disposable-binding.h

@@ -72,9 +72,6 @@ MRB_METHOD(disposableDispose)
 	if (!c)
 		return mrb_nil_value();
 
-	/* Inform core */
-	c->wasDisposed();
-
 	disposableDisposeChildren(mrb, self);
 
 	delete c;

src/disposable.h

@@ -30,6 +30,11 @@
 class Disposable
 {
 public:
+	~Disposable()
+	{
+		wasDisposed();
+	}
+
 	sigc::signal<void> wasDisposed;
 };