add backdrop and working menu with filtering

[maryx]

Adds:

• backdrop
• menu page
• filtering with onTap (except for My Account)
• uses ListViews instead of Columns for backdrop (yellow red errors otherwise)
• changes ProductColumn to Dart 2.0

Handoff to Will:

• Not sure if you want onDrag capability. The original Backdrop listens to when a user drags up. See onDragUpdate and onDragEnd here. Easy to port over if you want it. https://github.com/flutter/udacity-course/blob/master/unit_converter/unit_converter/lib/backdrop.dart
• Not sure what colors you wanted.