Owlkettle 2.0.0 is a declarative GUI framwork for GTK 4

[can-lehmann]

The `Property` widget in `custom_dialog.nim` is a simple viewable widget which 
is just an abstraction over the existing widgets. It is defined 
[here](https://github.com/can-lehmann/owlkettle/blob/main/examples/custom_dialog.nim#L29)
 and expands to a Box containing a Label.

You should be able to use a custom dialog without the `Property` widget like 
this:
    
    
    Dialog:
      Box:
        orient = OrientY
        Box:
          orient = OrientX
          Label(text = "Label 1")
        Box:
          orient = OrientX
          Label(text = "Label 2")
    
    
    Run