WelcomeScreen theming: Fix button border color

This change adds an additional color role for the border color of a
pressed button.

Task-number: QTCREATORBUG-13356
Change-Id: Ibdbd330f60063961dbf27dbe2fb90f32089d7b34
Reviewed-by: Thorben Kroeger <thorbenkroeger@gmail.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>