Add support for optional icon to Button (#3479)

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

crates/ui2/src/components/button.rs

@@ -1,4 +1,5 @@
 mod button;
+pub(self) mod button_icon;
 mod button_like;
 mod icon_button;