Add API for bringing the overlay as visually frontmost #168
Description
The bringToFront (naming tbd) API is needed by <vaadin-dialog>s upcoming modeless property (exposed from overlay).
The current overlay visual stacking order based on DOM order isn't enough by itself when there are more than one modeless dialogs open at once. Whenever a modeless dialog behind the currently frontmost dialog gets focused (by click for example), it's expected to be shifted as the new frontmost dialog.
An early implementation of the feature handles this by updating the DOM order of the dialog overlays, but that has a side-effect of active scroll positions of all content to get reset.
Visual stacking order should be changed to rely on z-index instead of DOM order.
The bringToFront function should then set the target overlay's z-index as the currently frontmost overlay's z-index + 1. It should also be invoked upon every overlay open making sure that any newly opened overlay gets displayed as the frontmost one.
Notification uses separate z-index space (on top of everything) & overlay implementation and thus shouldn't be affected by this change.