[NUI] Fix build warning for AccessibilityEvent naming (#2710)

[NUI] Fix build warning for AccessibilityEvent naming (#2710)

* [NUI] Fix build warning for AccessibilityEvent naming

- There's a build warning for View.Activate EventHandler : CS0108.
 'GaussianBlurView.Activate()' hides inherited member 'View.Activate'.
- Use the new keyword, 'Activated'.
- Update some code to follow formatting rules

Signed-off-by: Seoyeon Kim <seoyeon2.kim@samsung.com>
* [NUI] Add Accessibility prefix to events

- Add 'Accessibility' prefix to all Accessibility events.
- Fix build warning : CA2000
  Call Dispose on object created by signals before all references are out of scope.
  -> Make signal variable separately.

Signed-off-by: Seoyeon Kim <seoyeon2.kim@samsung.com>