This updates rb-c-button to include __icon and __label parts for

defining buttons containing both icons and labels in a consistent

manner. This will be styled with an appropriate gap in-between the two,

when both are present and visible.
Buttons now use flexbox for layout to enable this, and to ensure that

everything lines up correctly.
There are two new macros for customizing icons:


#rb-ns-ui.buttons.set-icon-only() will hide the label on a button

  and update the padding so the button looks correct with just an icon.


#rb-ns-ui.buttons.set-fa-icon() takes a FontAwesome icon variable

  and set it on the __icon part.

Tested with existing buttons, button groups, and menu buttons throughout

the UI.
Tested with some in-progress icon-only and icon-enabled buttons.