Assistant: Factor out web browser support.

Task-number: QTBUG-41130
Change-Id: Ia1d29b6eaa56e0f9eb569326e05f4c56411db6a7
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>
Assistant: Introduce DEFINES per browser type.

This makes it easier to add additional browser types and
switch between browsers.

Task-number: QTBUG-41130
Change-Id: I88e2890bfe9e815ed7deb9d867060f244b7e2a45
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>
Reviewed-by: Alejandro Exojo Piqueras <suy@badopi.org>