R1:4bdc51237a26

Add ability to have tooltips on buttons

Summary: Enables a basic tooltip when using icon buttons and a convenience method for setting an icon.

Test Plan: Built a UIExample.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12172