Polish the imageviewer example.

Polish the imageviewer example.

- Remove unneeded member variables.
- Use member initialization in the constructor.
- Use new connection syntax in createActions()
  to assemble the menu there, removing the createMenus()
  function.
- Fix coding style issue (braces)
- Avoid empty label showing up by setting the scroll
  area invisible until an image is loaded.
- Set a new image only if image read succeeds.
- Add status bar with information message.

Task-number: QTBUG-46848
Change-Id: I32d5af70d8eb71ec16dd58a0b98c32eb2bd988d7
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>