Opacity should be applied to all layers
Currently opacity is only set on the shape-group and not set on text, svg, etc. which is a bug.

While [discussing this](https://github.com/brainly/html-sketchapp/pull/83#discussion_r182401241) 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.