Add events for ui element visibility

## What problem does this solve or what need does it fill?

Currently when updating ui elements you need to do it whenever something related to it changes.

However this isn't always ideal, especially with more computationally expensive formulas.

## What solution would you like?

Add an `EnteredView`, and `ExitedView` events for ui elements

It may also be useful to add a `InView` component so that ui elements will only update when they are in view.

No files, tests, or entry points are named. Start by locating the existing UI element update and event/component mechanisms, then clarify the semantics of EnteredView, ExitedView, and the optional InView component; done should include agreed behavior and coverage for visibility transitions and update suppression.

Written by the indexing model from the issue text.