[NUI] Use weak reference for theme changed event

[NUI] Use weak reference for theme changed event

Previous theme changed event was holding reference of the view for the theme manager's lifetime
unless the view is disposed by calling Dispose() explicitly.
Since the theme manager is a static class, so there might be a change that the view can't get free by GC.
To solve this problem, this patch apply a weak event that uses weak reference for the handler
so that the views are no longer be caught by theme manager.

Signed-off-by: Jiyun Yang <ji.yang@samsung.com>