Providing image sizes with screenshots would allow better dynamic layout

Currently the click package index has two fields for screenshots - screenshot_url and screenshots_url [1]. They are set like:

  "screenshot_url": "https://myapps.developer.ubuntu.com/site_media/appmedia/2013/09/xkcd-1.png",
  "screenshot_urls": [
    "https://myapps.developer.ubuntu.com/site_media/appmedia/2013/09/xkcd-1.png"
  ],

(I'm guessing that screenshot_url is an old field and just screenshot_urls is what should be used in new code).

It's desirable when rendering these screenshots in a client (e.g. gnome-software) to know their size in advance. Ideally information like this would be available:

  "screenshots": [
    { "url": "https://myapps.developer.ubuntu.com/site_media/appmedia/2013/09/xkcd-1.png", "width": 800, "height": 1280 }
  ],

This allows us to allocate an appropriate amount of space for each screenshot then load them asynchronously.

[1] http://search.apps.ubuntu.com/docs/#package