Improve tooltip behavior for Application menu items

Summary:
Ref T12174.

• Fix text antialiasing pop-in at the end of the animation in Safari.
• Fix flickery animation replay when mousing inside a tooltip element, or rapidly switching between tooltip elements.
  • To accomplish this: don't play the animation if the last tip was hidden less than 500ms ago.

Test Plan:

• Viewed a tooltip in Safari.
• Waved cursor wildly over application items, both left-right (within an item) and up-down (between items).

Tooltips appeared stable and non-flickery in all cases.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12174

Differential Revision: https://secure.phabricator.com/D17272