Loading animation

The goal of this blueprint is to propose a generic way to visually notify of a pending operation in the UI (what's often referred to as "loading animation").

The current state of things is a glowing animation for buttons using a hardcoded animation on the background colour of the buttons (mostly used with list items). This animation is neither really visually satisfactory nor configurable.
We need to define a generic "loading state" visual indicator that can apply to buttons and other UI elements. We also need to take into account that while something is loading, we may want to disable (as in: grey out) some widgets.

At the moment, a good example of a part of the UI that lacks this kind of feedback is the screen containing a keyboard (e.g. the login screen or the search screen). When validated, the search or login operation generally takes some time, during which the user has no clue that something is actually happening, eventually restulting in some users clicking again or cancelling the process by accident.