[FIX] web: change selected field value on chain shortening

Before this commit, when opening a field chain with a field selector
and only removing parts of it, the field selector internal "chain"
value was properly updated but not its "selectedField" value.

addons/web/static/src/js/widgets/model_field_selector.js

@@ -240,6 +240,7 @@ var ModelFieldSelector = Widget.extend({
         if (this.pages.length <= 1) return;
         this.pages.pop();
         this.removeChainNode();
+        this.selectedField = this._getLastPageField(_.last(this.chain.split(".")));
         this.displayPage("o_animate_slide_left");
     },
     /// The goToNextPage method adds a new page to the popover following the given field relation and adapts