layout: clear page before unloading

This simplifies things for addins to only disconnect state in one place.

src/libide/layout/ide-layout-stack.c

@@ -438,6 +438,7 @@ ide_layout_stack_addin_removed (PeasExtensionSet *set,
   g_assert (plugin_info != NULL);
   g_assert (IDE_IS_LAYOUT_STACK_ADDIN (addin));
 
+  ide_layout_stack_addin_set_view (addin, NULL);
   ide_layout_stack_addin_unload (addin, self);
 }