Revert "Fixed collisions for shared tooltips."

This reverts commit 4b9df82d7d6f49dba49343a7d97ecc3cb8f1b113.
(Looks like it is better to keep mouseEnter stored in the tip data)
Used timestamps for collision detection.

The delayed close event checks to see that the tip has not been reshown
in the meantime. (It does this by comparing a timestamp "uid".)

This fixes the following problem: Hover over <a> show tip <t>. Now leave <a>
and quickly return to <a>. Tip closes when it should not.

(Problem is more pronounced when multiple elements show the same tip.)