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

src/applications/uiexample/examples/PHUIButtonBarExample.php

@@ -55,6 +55,7 @@ final class PHUIButtonBarExample extends PhabricatorUIExample {
         ->setTag('a')
         ->setColor(PHUIButtonView::SIMPLE)
         ->setTitle($text)
+        ->setTooltip($text)
         ->setIcon($image);
 
       $button_bar3->addButton($button);