This makes it a bit more approachable to implement.
This required converting the state of Component to AnyElement, which
costs an allocation. I'm hoping this will be ok performance-wise now
that AnyElements use bump allocation, but I need to benchmark still.
This PR extends `Button` with support for an optional icon to be
displayed next to the label.
As part of this, the functionality for displaying an icon within a
button has been factored out into an internal `ButtonIcon` component.
`ButtonIcon` is now used by both `IconButton` and `Button` to
encapsulate the concerns of an icon that is rendered within a button.
Release Notes:
- N/A
