Allow the target of the tooltip to be manually set through the target attribute to allow the tooltip to be anchored to the given element.

I have a somewhat specific problem in which I need to use a common paper-tooltip for multiple targets that are not siblings of the common paper-tooltip element. The reason for this design choice was to fix a bug where the tooltip was cut off when rendering inside of an iron-list element. The problem is best described by this Chromium bug: 870202, and the fix for this bug was implemented by the change 1176586. However, in the fix, the private _target property of the tooltip is directly accessed and assigned the target that is passed up to the parent of the common tooltip from the children.

Because of this bug, in my own implementation of a new element with similar structure, I have to do the same thing to prevent the tooltip from being cut off.

Who will use the feature?

Any developer that has a similar problem with the tooltip being cut off by other elements, or any developer who would like to use a common tooltip for targets that are children of siblings.

When will they use the feature?

This feature can be used when the target element is known, and it is not a sibling of the paper-tooltip element.

What is the user's goal?

To prevent the fix for the specific bug where the tooltip is being cut off from directly accessing a private property of paper-tooltip.