[grid] Add event to notify when the grid is visually ready

[vlukashov]

Description

As a developer I want to run some code after the grid contents is rendered. It would be awesome if there was a JS method returning a promise that resolves when all pending internal grid operations are completed, or a DOM event (or both). Currently the gird does not provide an API for that.

Example use cases:

• measure page load performance (grid is rendered => the page is rendered)
• start an animation after the grid is rendered
• script interactions with the grid in UI tests

Currently the grid exposes the notifying loading property, but that is not a good proxy for visual completeness:

Expected outcome

• a grid-loaded event is triggered
• grid.then() returns a promise

Browsers Affected

• [x] Chrome
• [x] Firefox
• [x] Safari
• [x] Edge
• [x] IE 11
• [x] iOS Safari
• [x] Android Chrome

[tomivirkki]

This should already be achievable by assigning CSS animations for the cell content and then listening to the animationstart events.