Improve QAxSelect dialog.

- Add filter widget from tools (slightly modified to contain
  the image as XPM). Add filter model, wire signals
  accordingly, add handling for the case none is selected
  (disabling the "Ok"-button).
- Use QDialogButtonBox instead of Buttons
- Remove obsolete context help button.
- Maintain "Binary compatibility" by replacing the QScopedPointer
  of the UI by a private class.

Change-Id: I1f4740cab6845525e0a71a3096feb2f2a4b2172a
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>