Identify: show most used observation fields like on obs show

[tiwane]

From: inaturalist/INaturalistIOS#613

On obs show, when you click in the Observation Fields field you see the last ten observation fields you used (I think) and can easily select one of them wihtout typing anything in.

But on Identify, clicking on the Obseration Field field in the Annotations section of the model shows you nothing - you have to start typing. Request is to make this field act like it does on obs show.