Add keywords member to the manifest

[filips123]

Similar to already existing categories member, keywords can also be used to describe the site/app. However, unlike categories, they should not be some pre-defined words, but describe the site in depth with arbitrary words, similar to old meta keywords.

Because sites could abuse this and add a lot of unrelated keywords, catalogs and stores should not be required to honor this hint. I know meta keywords were abused a lot and now not many search engines honor them, but I think that with manifest keywords this wouldn't introduce such a big problem, because:

1. Categories and description fields can already be to manipulate results.
2. I think most stores have some manual or automated review before publishing the app that could catch categories, keywords, and description abuse.

Keywords could then be used to provide more details about the site. They could be displayed to the user similarly to categories and could also be used to provide additional search words for which the site should be displayed.