Restrict font-smoothing to actual buttons

Summary: We use font-smoothing to make better looking buttons, but it makes text buttons look bad.

Test Plan: Test a button in an actionlist, font looks same as other action list items.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11776