simple-overlay demo

### An element with `IronOverlayBehavior` can be opened, closed, toggled.

### Use `with-backdrop` to add a backdrop to your overlay. Tabbing will be trapped within the overlay.

### Use `restore-focus-on-close` to return the focus to the element that opened the overlay when it gets closed.

### The child with `autofocus` gets focused when opening the overlay.

### Multiple overlays can be opened.

### Use `always-on-top` to keep the overlay on top of others.

### An element with `IronOverlayBehavior` can be scrollable or contain scrollable content.