When using url go properties, list the objects in the outliner in a combo box

**Is your feature request related to a problem? Please describe (REQUIRED):**
If i change the name of the node in the outliner i need to update the name in the script too.

**Describe the solution you'd like (REQUIRED):**
Imagine that we have a way to define properties on guis/gos that accepts an url.

gui.property('btn_ok', url())
go.property('player', url())

When I see that property on the properties panel I would see a combo box with all of the current nodes in the outline so I can pick one.

I don't know how that would work with collection proxies, maybe show a "ghost" version of the collection that is going to be instantiated so we can pick parent nodes too?

**Describe alternatives you've considered (REQUIRED):**
I can't think of an alternative