Better looking widget toolbox in Designer

I noticed that in many styles the widget toolbox looks rather
ugly when using the styled buttons. Instead I now draw simpler
and cleaner expander buttons.

Change-Id: I3559c8ffcae3e08a7b418c4238663af579bcc912
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>