[BUG] Autoupdate functionality should clean up after itself.

[jgerigmeyer]

The autoUpdate function used to update positioning when DOM elements move (or on scroll/resize) returns a cleanup function that should be called to remove observers.

It’s important that this function is only called/set-up when the floating element is open on the screen, and cleaned up when it’s removed. Otherwise, it can cause severe performance degradation, especially with many floating elements being created.

This is related to #91 (and partially implemented in #123).