Opacity should be applied to all layers

[kdzwinel]

Currently opacity is only set on the shape-group and not set on text, svg, etc. which is a bug.

While discussing this we also found out that we don't calculate opacity properly. 'opacity' property is non-cascading in CSS, but it's effect does "cascade" on children (e.g. <div opacity=.5><div opacity=.5></div> - inner div should have opacity of 0.25). In order to fix that we should collect all parent opacities and multiply them.

[kdzwinel]

It turns out to be a bit more work because we need to add opacity support to SVG and TextStyle objects.