Move spark-menu-button back to spark-polymer-ui + improve file filter similar to #1953

@dinhviethoa

This simplifies things quite a bit and keeps the UI almost fully separated from the business logic and enclosed in a single top-level Polymer element (which gives a lot of niceties in controlling the UI). This will also be important for unified action management in the UI that I'm planning to implement.

Also implemented improvements in the file filter behavior similar to #1953.