Disable expand behaviours when the list is empty

[leobia]

Fixes #228

Side note: The termometer Thing have the arrow enabled even if it shows nothing because the thing.behaviours list contains an object. Here a console log of the thermometer behavior {"@class":"com.freedomotic.model.object.RangedIntBehavior","name":"temperature","description":"","active":true,"priority":-1,"readOnly":true,"value":0,"max":1000,"min":-600,"scale":10,"step":1}

[mcicolella]

Ok. The thermometer has a "readOnly" behaviour. Maybe we should disable the control when the list is empty or it contains exclusively "readOnly" behaviours.

[leobia]

The expand toggle is now disabled when the behaviours list is empty or contains only readOnly items

[mcicolella]

Great work, thanks!