MX LinkButton not as advertised

From Royale docs: "The LinkButton control is a borderless Button control whose contents are highlighted when a user moves the mouse over it."

At least with just an icon, it has a border _and_ doesn't have a rollover highlight (implemented as a background color change, in Flex).

Workaround for border, of course, is ``style="border: 0;"``.

Also need to check on default horizontal alignment (middle) if icon is smaller than width.

Start with the Royale documentation quoted in the issue and locate the LinkButton control implementation. Reproduce the icon-only case, then inspect its border, rollover background behavior, and default horizontal alignment. Done means the control matches the documented borderless and rollover behavior, with alignment checked for a smaller icon.

Written by the indexing model from the issue text.