Highlight colour should blink when a flat widget is activated

## Problem
Users who mainly use keyboards to select items, or have already selected an item via cursor, will not get any visual feedback as to whether they have activated the item when they click it. A good example of this could be selecting an item in Files and then clicking it - there is no indication to re-affirm the user of what they just did "The user is drunk, even if they actually aren't"

## Proposal

The accent colour should blink or pulse (a bit like macOS?) when a flat widget is interacted with. This isn't really necessary for toggles or buttons since they are highlighted with a border rather than a backdrop and animate when activated.

## Prior Art

#### Single blink:

https://user-images.githubusercontent.com/58219504/106761513-cdaa0d00-662c-11eb-9ec9-172dbc5782aa.mov

#### Double blink:

https://user-images.githubusercontent.com/58219504/106761631-ee726280-662c-11eb-8f20-a5ca82655633.mov