Draft: Feature - Group Translucency

Review requested

This feature puts new code into SPDesktop, please review to make sure that this makes sense.

Still to do:

• Add preference to allow this feature to be optional
• Add preference watcher to make it update properly
• Fix interaction with the objects dialog
• Fix color picker

When entering a group that is not a layer, we turn the opacity down for all objects outside of that group to make it clear that we are inside a group and which one and which objects we're working with.