Implement feature-app-links to support in page app banners.

[jhugman]

Connected: #2936

This feature component feature-app-links should listen to the URL changing in a session and attempt to open in as a native app, or launch a fallback, or launch the Play store.

This should only be the result of a user interaction.

This is to support web pages that implement their own app banners.

The fallback is implemented thus:

• Checks to see if it's a URL scheme that Fennec can handle — do so here.
• If not, then parses the custom:// URLs based upon schemes only and checks it against the OS scheme handlers
• if not, then fallbacks if no app deals with that scheme, either by going to the market place or a fallback URL supplied in the URL.
• Finally, checking the time since last user interation (as a proxy for whether the user clicked on the link), then fail silently or display an unknown protocol error.

┆Issue is synchronized with this Jira Task

[jhugman]

Fixed by #3058.