AppD schema does a poor job of validating icons and screenshots

[kriswest]

Area of Issue

[x] App Directory

Issue Description:

Icons in app appD records should have a format like:

...,
"icons": [
    "src": "<some URL>,
    "size": "25x45",
    "type": "image/png"
],
...

(the last two fields are optional, the src field should be required)

However, the schema is poorly written in that it is missing the required property declaration and does not preclude additionalValues. Hence, an entry such as:

...,
"icons": [
    "url": "<some URL>,
],
...

will validate as src was not correctly required in the schema and additional properties are allowed.

Refine the schema so that it can better validate appD records.