Implementation of the button for QML-based dialogs matches
the one for QML examples.  The button and the text field
use system palette colors, as does the file dialog.

Change-Id: Ibf45d57bdab8799ae6aa69ba543c0e05c55b01d3
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>