Polish the screen shot example.

- Remove unneeded member variables.
- Set window sizes depending on screen geometry for High DPI screens.
- Flesh out code for saving the image, setting the supported
  mime types on the file dialog.
- Streamline constructor code, remove create...() functions.
- Use new connection syntax in createActions(),
- Obtain the screen from the widget.
- Adapt documentation. Remove note saying that widgets do not need
  the parent parameter (since creating parentless widgets can
  result in flicker in some cases), explain that QScreen pointers
  should be checked.

Change-Id: I0332bbf10eafe861fe3fd5573522694ab5c0183a
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>