Add support for iMessage app icons

[neonichu]

[seanlabastille]

As a heads up, I had a quick look into how doable this would be.
It might be worth combining this with support for other uses of asset catalogs as there are some interesting combinations of properties which may require more extensive refactoring.
As well as non-square images, etc.

Just to get an idea of what is roughly required, this is where I was going with needed_icons:

imessage: {
            iphone: {
              '2x' => ['60x45'],
              '3x' => ['60x45']
            },
            ipad: {
              '1x' => ['67x50'],
              '2x' => ['67x50', '74x55']
            },
            universal: {
              '1x' => ['27x20', '32x24'],
              '2x' => ['27x20', '32x24'],
              '3x' => ['27x20', '32x24']
            },
            'ios-marketing' => {
              '1x' => ['1024x768']
            }
          },