Make nuxeo-actions-menu use ResizeObserver

nuxeo-action-menu has some limitations with regards to responsiveness, because it relies solely on Polymer's IronResizableBehavior. For example, if we dynamically change it's width, it will fail to rerender, forcing one to manually call notifyResize(). We should make use of ResizeObserver instead, to rebalance the actions when the element's dimensions change.