Don’t use weak pointers with NSViewController

This is not supported on 10.7 (and leads to a crash).

Frameworks/OakFilterList/src/datasources/BundleItemChooser.mm

@@ -85,7 +85,7 @@ OAK_DEBUG_VAR(FilterList_BundleItemChooser);
 @end
 
 @interface BundleItemChooserView : NSView
-@property (nonatomic, weak) BundleItemChooserViewController* viewController;
+@property (nonatomic, assign) BundleItemChooserViewController* viewController;
 @end
 
 @implementation BundleItemChooserView