Allow specifying icons for buttons

[mturoci]

Is your feature request related to a problem? Please describe

Additional icons improve general UX and visuals.

Describe the solution you'd like

icon prop on ui.button

[lo5]

What would the buttons look like visually in practice?

[mturoci]

from https://developer.microsoft.com/en-us/fluentui#/controls/web/button

[derrick56007]

this would be very useful

[lo5]

@mturoci - I didn't realize Fluent is already capable of this. Agree - icon on ui.button() would be a no-brainer.

[mturoci]

Tracked in #747.