Enhance favicon support

[benfrancis]

Currently Shell just uses the first icon in the list provided in the page-favicon-updated event by Electron. This is because Electron only provides an array of URLs without any additional metadata to choose between them. The first icon may not necessarily be the best size and type to display, or even be a supported image format.

Enhancing this function requires modifying Electron, or possibly Chromium. See electron/electron#14803