Add opacity to the group element

[chrfalch]

As per the Svg 1.1 reference, a group or container element can have an opacity setting:

14.5 Object and group opacity: the ‘opacity’ property

I'd like to implement support for group opacity by:

• Adding support for opacity as a short for fill-opacity (will affect both pen and brush) on elements
• Adding support for multiplying the group's opacity with its children's pen/brush opacity.

Solution:

• To avoid changing too many classes we can let the Group object visit its children before and after drawing to change its children's alpha values (pens and brushes).
• Visiting children should not be performed if no opacity is set on the group.
• Subsequent assignments to the group's opacity property should not result in alpha values growing in either direction.

[praeclarum]

You're my hero! Thanks for the tests, it's very much appreciated.